new chronological and geochemical constraints on the genesis and geological evolution of ponza and palmarola volcanic islands tyrrhenian sea italy abstract a new geochronological and geochemical study of the volcanic rocks of the ponza and palmarola islands pontine archipelago has been carried out this archipelago is located along the boundary between the italian continental shelf and the opening tyrrhenian basin it is a key area to study volcanism related to the opening of the tyrrhenian sea ponza is the oldest felsic magmatic manifestation in the central tyrrhenian area previous studies suggested that ponza volcanic activity began before 5 ma twenty five new k ar ages constrain the volcanic activity rhyolitic hyaloclastites and dykes to the last 4 2 ma with two episodes of quiescence between 3 7 and 3 2 ma and between 2 9 and 1 0 ma a new volcanic episode dated at 3 2 2 9 ma has been identified on the central and southern ponza with emplacement of pyroclastic units at 1 0 ma a trachytic episode ended the volcanic activity the near island of palmarola exhibits rhyolitic hyaloclastites and domes dated between 1 6 and 1 5 ma indicating that the island was entirely built during the early pleistocene in a short span of time of ca 120 ka although only 6 8 km apart the two islands display significantly different geochemical signatures ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction collision zones high k calc alkaline and metaluminous rhyolites agpaitic index ai and alumina saturation index asi 3 and nb ta negative anomalies in palmarola the orogenic character is also present but much less marked than in ponza rhyolites have a peralkaline character ai 1 lower lile hfse th ta 11 15 low lree hfse ratios la nb 1 2 close to those of anorogenic lavas and the nb ta negative anomalies are almost absent y nb ratios indicate different magmatic sources one similar to island arc or active continental margin basalts for ponza rhyolites and the others probably involving an oib type component for palmarola rhyolites and ponza trachytes palmarola volcanics represent a transitional magmatism although a preserved collisional geochemical imprint they show geochemical features approaching those of anorogenic lavas erupted in a within plate context the change of magmatism evidenced in this study can be related to the tectonic evolution of the area indeed hf ta and rb contents suggest that the oldest pliocene rhyolites of ponza would emplace in a syn to late collisional setting while the younger pleistocene rhyolites of palmarola would be emplaced in a post collisional setting in which the orogenic character th ta decreases and mantle influence nb ta increases geochemical modeling strongly suggests that the palmarola rhyolites represent the waning stages of a subduction related magmatism the k ar datings allow us to estimate precisely the transition of magmatism to last less than 1 3 ma the transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources this upwelling could occur during the extension of the tyrrhenian basin caused by the slab retreat and steepening or during a process of slab break off starting in the pliocene

petrology and isotope geochemistry of san vincenzo rhyolites tuscany italy two groups of rhyolites have been recognized at san vincenzo tuscany italy group a rhyolites are characterized by plagioclase quartz biotite sanidine and cordierite mineral assemblages they show constant mgo and variable cao and na2o contents initial87sr 86sr ratios in group a samples range between 0 71950 and 0 72535 whereas the nd isotopic compositions are relatively constant 0 51215 0 51222 group b rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals and show textural mineralogical and chemical evidence of interaction with more mafic magmas the sr and nd isotopic ratios range between 0 71283 0 71542 and 0 51224 0 51227 respectively magmatic inclusions of variable size 1 mm to 10 cm were found in groups b rhyolites these inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite they are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts these magmatic inclusions have relatively high sr contents 996 1529 ppm and sr and nd isotope ratios of 0 70807 0 70830 and 0 51245 0 51252 respectively 87sr 87sr data on minerals separated from both group a and b rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group a rhyolites and due to interaction of rhyolites with a mantle de rived magma in group b rhyolites isotopic data on whole rocks and minerals allow us to interpret the group a rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source conversely group b rhyolites underwent interactions with a mantle derived magma the crustal source as inferred from isotopic systematics would be characterized by87sr 86sr and143nd 144nd ratios close to 0 7194 and 0 51216 respectively the sub crustal magma would have sr isotopic composition close to 0 7077 and a143nd 144nd ratio greater than or equal to 0 51252 these isotopic features are different from those reported for the parental magmas postulated for vulsini and alban hills in the nearby roman magmatic province and are similar to those of the vesuvius and ischia magmas

geothermometry geochronology and mass transfer associated with hydrothermal alteration of a rhyolitic hyaloclastite from ponza island italy abstract a rhyolitic hyaloclastite from ponza island italy was hydrothermally altered producing four distinct alteration zones based on x ray diffraction mineralogy and field textures 1 nonpervasive argillic zone 2 propylitic zone 3 silicic zone and 4 sericitic zone the unaltered hyaloclastite is volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli incomplete alteration of the hyaloclastite resulted in the nonpervasive argillic zone characterized by smectite and disordered opal ct the other three zones exhibit more complete alteration of the hyaloclastite the propylitic zone is characterized by mixed layer illite smectite i s with 10 to 85 i mordenite opal c and authigenic k feldspar akspar the silicic zone is characterized by i s with 90 i pure illite quartz akspar and occasional albite the sericitic zone consists primarily of i s with 66 i pure illite quartz and minor akspar and pyrite k ar dates of i s indicate hydrothermal alteration occurred at 3 38 0 08 ma oxygen isotope compositions of i s systematically decrease from zones 1 to 4 in the argillic zone smectite has 18o values of 21 7 to 22 0 and i s from the propylitic silicic and sericitic zones ranges from 14 5 to 16 3 12 5 to 14 0 and 8 6 to 11 9 respectively 18o values for quartz from the silicic and sericitic zones range from 12 6 to 15 9 by use of isotope fractionation equations and data from authigenic quartz hosted primary fluid inclusions alteration temperatures ranged from 50 to 65 c for the argillic zone 85 to 125 c for the propylitic zone 110 to 210 c for the silicic zone and 145 to 225 c for the sericitic zone fluid inclusion data and calculated 18owater values indicate that hydrothermal fluids were seawater dominated mass transfer calculations indicate that hydrothermal alteration proceeded in a relatively open chemical system and alteration in the sericitic zone involved the most extensive loss of chemical species especially si systematic gains in mg occur in all alteration zones as a result of i s clay mineral formation and systematic losses of na ca and k occur in most zones with the exception of ca calculations of mass transfer associated with hydrothermal alteration on ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater the anomalous ca loss at ponza may be due to hydrothermal formation of anhydrite and later low temperature dissolution on the basis of mg enrichments derived from circulating seawater we estimate the following minimum water rock ratios 9 3 6 and 9 for the argillic propylitic silicic and sericitic zones respectively hydrothermal fluid ph for the propylitic and silicic zones was neutral to slightly basic and relatively acidic for the sericitic zone as a result of condensation of carbonic and perhaps other acids

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

isostatic state and crustal structure of north china craton derived from goce gravity data abstract the crustal structure between different subregions of the north china craton ncc plays an important role in improving our understanding on the craton evolution here we present a flexural moho map and isostatic residual gravity field of the ncc derived from the go cons gcf 2 tim r3 model we first investigate the influence of additional sources on the forward flexure calculation especially the distinct thinning of the lithospheric thickness from west to east after removing the gravity effects of the lab undulations the flexural moho and isostatic residual gravity field of the ncc are calculated using the vening meinesz regional isostasy models there are high to medium positive residual anomalies beneath the eastern block and trans north china orogen while negative residual anomalies delineate the yinshan block ordos plateau and khondalite belt this obvious disparity is consistent well with the decreasing depth of the flexural moho from west to the east a profile across the eastern block trans north china orogen and the western block was employed to construct a density structure of the crust and uppermost mantle by comparing the surface topography bouguer gravity isostatic moho and seismological moho undulations we find that the flexural moho is much shallower than the seismic moho beneath the northern margin of the ordos block possibly indicating the over compensated isostatic state caused by the dominant nw se oriented compressional deformation in the lithospheric processes density modeling results further predicted that a high density body may exist in the lower crust beneath the northern margin of the ordos block which could be a westward dipping upper middle crust remnant stagnating in the lower crust beneath the western block medium to high pressure metamorphism of granulite facies or eclogite facies resulting from subsequent continental thickening during the continental collision may be responsible for modifying the remaining subduction remnant

multiple mantle metasomatism in central southern italy geochemical effects timing and geodynamic implications the regional petrological and geochemical variations observed in the recent magmatism along the italian peninsula are interpreted to testify to the coexistence of distinct sectors of upper mantle these are suggested to result from at least three compositionally and temporally distinct metasomatic events that affected petrologically different premetasomatic mantle sources geological and geochemical evidence suggests that metasomatism in the northern sector occurred during the alpine subduction by introduction of a composition similar to the deeply subducted dora maira metagranites into a residual lithospheric mantle of upper crustal material metasomatic events beneath the roman and neapolitan areas are younger these are related to the addition of melts and fluids into a fertile asthenospheric mantle during the latest stage of the west directed apennine subduction of the adria plate and during the current northwest subduction of the ionian sea floor

application of 40ar 39ar and 230th dating methods to the chronostratigraphy of quaternary basaltic volcanic areas the ustica island case history abstract the island of ustica has been the site of coeval volcanism and transgressive regressive marine cycles the volcanic history can be subdivided into five periods of activity according to stratigraphical and compositional data basaltic magmas were extruded during most of these periods and only one period was characterized by the emplacement of trachytic rocks sea level high stands generated five orders of marine terraces with related level surfaces at variable elevation and sedimentary deposits a geochronological study was carried out by means of 40 ar 39 ar and 230 th dating methods on magmatic and marine sedimentary rocks respectively the results obtained allow us to give temporal constraints for the geological history of the island volcanism took place between about 750 and 130 ka bp until about 500 ka bp ustica was a seamount with subaqueous eruptions the sole trachytic explosive eruption occurred about 424 ka bp after a long period of rest during which the conditions for formation and evolution of a shallow magma chamber were established the ages of the marine terraces are in very good agreement with those obtained for correlatable level surfaces along the coasts of the southern tyrrhenian sea the i order marine terrace 80 120 m a s l was created about 350 ka bp while the iii order terrace 20 30 m a s l which is the youngest dated formed around 130 ka bp according to geomorphological constraints the iv 10 m a s l and v order terraces 5 m a s l are correlated with level surfaces dated at 105 and 80 ka bp

the role of slab detachment processes in the opening of the western central mediterranean basins some geological and geophysical evidence abstract a review of the geological and geophysical data from the central and western mediterranean region and the present day upper mantle structure derived from tomographic studies are utilized in order to define the oligocene recent geodynamic evolution for the area in line with previous work we suggest that the miocene quaternary opening of the western and central mediterranean basins is the result of back arc extension due to the roll back toward the southeast of a northwestward subducting african slab in a geodynamic setting pinned between the alpine and betic collisional zones we find however that this general pattern is complicated by four different detachment events which occurred beneath the alps early oligocene the betic chain aquitanian northern africa langhian and the apennines late miocene pliocene we show that each of these events determines a major tectonic reorganization within the european plate

structure and geological evolution of the island of ponza italy inferences from geological and gravimetric data abstract a geological and a geophysical survey have been carried out at ponza island tyrrhenian sea italy geological and structural data allowed to identify three main tectonic systems nw se ne sw and e w trending the first one is related to a pre volcanic tectonic event probably linked to the pliocene extensional activity of the tyrrhenian evolution the other two systems affected the volcanic units in two different stages of the lower pleistocene the earlier one after the rhyolitic hyaloclastic formation hf emplacement and the later one after the emplacement of older trachytic pyroclastic deposits lower pyroclastic units lpu the latter event was followed by the emersion of the whole ponza area as testified by a marked erosional surface and marine terrace deposits cropping out at the top of lpu the upper pyroclastic units upu represent the younger trachytic activity of the island 1 3 myr and do not show evidence of tectonic activity the nw se trending tectonic system probably assisted the rhyolitic magma rise while the ne sw and e w trending systems mainly assisted the trachytic magma rise responsible for the explosive and effusive activity in the southern area and for the hydrothermal fluids that caused alteration processes in the northern area a 161 station gravimetric survey was carried out on the island and surrounding islets the geological data and the gravimetric survey have been used to propose a 2 5 d model in which rhyolitic hyaloclastic deposits 1 7 g cm 3 overlay an articulated meso cenozoic sedimentary substratum 2 6 g cm 3 laying at a depth to 300 m below sea level both formations are crossed by rhyolitic dykes 2 4 g cm 3 which mark feeder fractures in the m guardia area where a maximum is present this model accounts for the presence of a horst of the rigid basement a shallow trachytic lava flow and its feeder 2 8 g cm 3

lithospheric boudinage in the western mediterranean back arc basin the western mediterranean is composed of irregular troughs formed as back arc basins in the hanging wall of the w directed apenninic subduction which retreated eastward during neogene and quaternary times the basins are progressively younger toward the east ageing from late oligocene early miocene valencia provencal alboran and algerian basins to middle miocene pleistocene tyrrhenian sea the basins isolated boudins of continental lithosphere the sardinia corsica block representing the largest the boudinage has a wavelength of 100 400 km and facilitated stretching of the continental lithosphere with formation of new oceanic crust in the provencal algerian vavilov and marsili basins the boudins developed both in the earlier alpine betic orogen alboran basin and in its foreland provencal and valencia troughs the extension appears clearly asymmetric due to its eastward polarity accommodated by e dipping master low angle normal faults moreover the thinning shows variations in boudinage wavelength and is characterized by several along strike transfer zones and heterogeneities the western mediterranean back arc setting is comparable with atlantic and western pacific back arc basins associated with w directed subduction zones that show similar large scale lithospheric boudinage

facies architecture and origin of a submarine rhyolitic lava flow dome complex ponza italy the islands of ponza and gavi western pontine archipelago italy preserve parts of a subaqueously emplaced high silica 75 77 sio2 rhyolitic lava flow that overlies the margins of three older domes exposures of the lava alone cover 24 km2 and with a thickness of 150 m exposed in the cliffs the preserved volume of the lava flow is 3 6 km3 and the original volume is likely to have exceeded 7 km3 the lava flow consists of coherent obsidian and several hyaloclastite facies including in situ and clast rotated breccias and sandstones which all exhibit gradational relationships the coherent obsidian occurs as separate domains and is often flow banded although flow folds occur the banding is generally subhorizontal dipping gently to the nne the hyaloclastite representing 90 of the lava by volume has a diffuse layering varying from 0 5 to 2 m in thickness this layering also dips gently to the nne and is marked in places by alternation of some coherent horizons coarse and finely fragmented in situ hyaloclastite pumiceous hyaloclastite and clast rotated hyaloclastite the layering is laterally continuous and consistent in orientation over most of the 13 km length of ponza the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow the resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones the lava flow has been intruded and crosscut by rhyolitic bodies of various ages including discordant coherent to internally fractured contemporaneous rhyolite bodies and late penecontemporaneous dikes with glassy margins and alteration haloes

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

upper mantle domains beneath central southern italy petrological geochemical and geophysical constraints the italian peninsula shows high complexity of the mantle crust system and of the plio quaternary magmatism the lithospheric thickness has remarkable lateral variations from about 110 km to about 30 km intermediate and deep focus earthquakes indicate the presence of a lithospheric slab under the aeolian calabrian area and at the southern end of campania much less extensive intermediate depth seismicity characterizes the roman tuscany region where the existence of a relic slab has been hypothesized the deep seismicity in the southern tyrrhenian sea is associated with active calcalkaline to shoshonitic volcanism in the aeolian arc alkaline potassic volcanism occurs in central italy and potassic lamproitic magmatism coexists with crustal anatectic and various types of hybrid rocks in the tuscany area the parallelism between changing magmatism and variation of the structure of the crust mantle system makes central southern italy a key place where petrological and geophysical data can be used to work out an integrated model of the structure and composition of the upper mantle beneath tuscany the upper mantle has been affected by intensive subduction related metasomatism this caused the formation of phlogopite rich veins that cut through residual spinel harzburgite and dunite these veins possibly partially molten may explain the unusually soft mechanical properties that are detected just below the moho in the roman province the upper mantle is formed by a relatively thin lid the mantle part of the lithosphere and by metasomatic fertile peridotite probably connected with the upraise of an asthenospheric mantle wedge above the apennines subduction zone geochemical data indicate that metasomatism though still related to subduction had different characteristics and age than in tuscany in the eastern sector of the aeolian arc and in the neapolitan area the upper mantle appears to be distinct from the roman and tuscany areas and is probably formed by fertile peridotite contaminated by the presently active subduction of the ionian sea floor the overall picture is that of a mosaic of various mantle domains that have undergone different evolutionary history in terms of both metasomatism and pre metasomatic events the coexistence side by side of these sectors is a key factor that has to be considered by models of the geodynamic evolution of the central mediterranean area

melt inclusion geochemistry and computer modeling of trachyte petrogenesis at ponza italy abstract the punta della guardia trachytic unit on the island of ponza italy is thought to represent the latest stage in the evolution of a parent basaltic melt in a shallow magma chamber undergoing fractional crystallization to test this hypothesis melt inclusions mi have been analyzed to constrain the crystallization history of the ponza trachyte the trachyte is a weakly porphyritic lava that contains phenocrysts of plagioclase k feldspar clinopyroxene and biotite melt inclusions were observed and measured in plagioclase clinopyroxene and k feldspar the crystallized devitrified melt inclusions were heated to dissolve melt the crystals quenched to a homogeneous glass and then analyzed for major elements by electron microprobe two different heating protocols one involving rapid heating in a one atmosphere furnace and the other involving slower heating in a microscope heating stage were used to homogenize the inclusions in order to investigate possible effects of heating rates on inclusion composition no detectable difference was found melt inclusion compositions suggest that the magma evolved from an early phase dominated by clinopyroxene plagioclase crystallization to a late stage in which melt composition was controlled by precipitation of two feldspars the potential basaltic parent magma does not crop out on ponza although more primitive and coeval basalts and trachybasalts are found on nearby ventotene island which is considered to be co magmatic with ponza using the program melts we have compared results obtained from melt inclusions in the ponza trachyte with melt compositions predicted for fractional crystallization of a magma having the composition of the ventotene trachybasalt crystallization trends predicted by melts are consistent with data from melt and host phases although differences in measured and predicted compositions are significant in some cases in spite of these differences the model results and measured compositions are consistent with the hypothesis of a genetic link between the magmatism of ponza and ventotene

constraining the source regions of lunar meteorites using orbital geochemical data lunar meteorites provide important new samples of the moon remote from regions visited by the apollo and luna sample return missions petrologic and geochemical analysis of these meteorites combined with orbital remote sensing measurements have enabled additional discoveries about the composition and age of the lunar surface on a global scale however the interpretation of these samples is limited by the fact that we do not know the source region of any individual lunar meteorite here we investigate the link between meteorite and source region on the moon using the lunar prospector gamma ray spectrometer remote sensing data set for the elements fe ti and th the approach has been validated using apollo and luna bulk regolith samples and we have applied it to 48 meteorites excluding paired stones our approach is able broadly to differentiate the best compositional matches as potential regions of origin for the various classes of lunar meteorites basaltic and intermediate fe regolith breccia meteorites are found to have the best constrained potential launch sites with some impact breccias and pristine mare basalts also having reasonably well defined potential source regions launch areas for highland feldspathic meteorites are much less well constrained and the addition of another element such as mg will probably be required to identify potential source regions for these

k ar dating extended into the last millennium application to the youngest effusive episode of the teide volcano spain less than one thousand years dating of the youngest volcanic episode of the teide volcano canary islands spain has been performed using the k ar cassignol technique analyses of about 20 g of pure alkali feldspar yielded a weighted mean age of 800 300 years stratigraphic historic and archeomagnetic dating validate this age and rule out the hypothesis of the presence of significant excess argon contamination for this flow our result shows that the last effusive activity of the teide volcano took place shortly before european settlement in tenerife island this study confirms that the cassignol technique can be applied with success for historic dating and demonstrates that it can now even be extended to the last millennium

behaviour of trace elements during magmatic processes in the crust application to acidic volcanic rocks of tuscany italy abstract plio quaternary acidic rock suites from three volcanic centres of tuscany italy san vincenzo roccastrada and monte amiata are composed predominantly of rhyolitic and rhyodacitic ignimbrites the rocks are characterized by distinct enrichments particularly of k and rb and display considerable compositional differences among the various volcanic centres the volcanic rocks of roccastrada and most of s vincenzo were probably generated by partial melting of metapelitic rocks from the paleozoic basement of tuscany leaving a residue composed of biotite cordierite and quartz and sometimes plagioclase or garnet the enrichment of k and related elements can be attributed to selective contamination by fluids the other volcanic rocks including trachytes of mt amiata have also been affected by interaction with an upper mantle component probably k rich basaltic rocks a similar petrogenetic model can be applied to the granodiorite pluton of mt capanne elba which belongs to the same magmatic cycle since the variation trends observed in the tuscany volcanic rocks are similar to those reported from many acidic plutons it appears that crustal anatexis accompanied by interaction with upper mantle components has frequently played an important role in the genesis of the granitic rocks

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

hydrothermal alteration of a rhyolitic hyaloclastite from ponza island italy abstract a rhyolitic hyaloclastite from ponza island italy has been hydrothermally altered producing four distinct alteration zones based on xrd and field textures 1 non pervasive argillic zone 2 propylitic zone 3 silicic zone and 4 sericitic zone the unaltered hyaloclastite is a volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli incomplete alteration of the hyaloclastite resulted in the non pervasive argillic zone characterized by smectite and disordered opal ct obsidian clasts some pumice lapilli and pyrogenic plagioclase and biotite are unaltered smectite has an irregular flakey morphology although euhedral particles are occasionally observed the propylitic zone is characterized by mixed layer illite smectite i s with 10 to 85 illite i mordenite opal c and authigenic k feldspar akspar the matrix of the hyaloclastite is completely altered and obsidian clasts are silicified however plagioclase and biotite phenocrysts remain unaltered flakey i s replaces pumice and mordenite akspar and silica line and fill pores i s particles are composed predominantly of subequant plates and euhedral laths the silicic zone is characterized by highly illitic i s with 90 i quartz akspar and occasional albite in this zone the matrix and clasts are completely altered and pyrogenic plagioclase shows significant alteration illitic i s has a euhedral lath like morphology in the sericitic zone the hyaloclastite altered primarily to illitic i s with 66 i quartz and minor akspar and pyrite clay minerals completely replace pyrogenic feldspars and little evidence remains of the original hyaloclastite texture unlike other zones illitic i s is fibrous and pure illite samples are composed of euhedral laths and hexagonal plates the temperatures of hydrothermal alteration likely ranged from 30 to 90 c for the argillic zone from 110 to 160 c for the propylitic zone from 160 to 270 c for the silicic zone and were possibly as high as 300 c for the sericitic zone the four zones occur as linear bands that increase in intensity north of the bentonite mine at cala dell acqua the alteration zones have two orientations and may be structurally controlled by e w and ne sw trending faulting which is consistent with the dominant structural trends of the pontine archipelago finally hydrothermal alteration most likely involved seawater based on the geologic evolution of ponza

seismic study of the 41st parallel fault system offshore the campanian latial continental margin italy abstract a set of seismic reflection lines collected in the southern tyrrhenian sea offshore the campanian latial continental margin has been selected and interpreted the aim is to characterize 1 the structural features of the 41st parallel line 41pl an e w elongated magnetic anomaly zone separating the northern tyrrhenian domain from the southern tyrrhenian domain and 2 to study the relationships between the 41pl and the ortona roccamonfina line orl a tectonic structure transversal to the apennines along which the northern apennine arc merges the southern arc the interpretation of the seismic lines is discussed in light of the available geological stratigraphic and structural and geophysical gravimetric and magnetic information results show that the campanian latial continental margin is characterized by a series of structural lows and highs that match with the main structures on the mainland mt massico horst the volturno and garigliano depressions the study area is characterized by ese wnw to e w and ne sw striking faults the activity of these faults developed during pliocene early pleistocene times ese wnw to e w faults display structures consistent with strike slip movements these faults which are located on the maximum gradient of the e w elongated magnetic alignment of 41pl are responsible for the sse translation of the offshore sector of the mt massico horst the horizontal dislocation of mt massico suggests left lateral movements for the ese wnw to e w faults the ne sw faults that affect the mt massico horst which represents the southern tip of orl show seismic features consistent with normal movements as also revealed by inland data since 1 background seismicity is virtually absent along the study area and 2 the uppermost seismic reflectors seem unaffected by faults it is very likely that both the orl and 41pl fault zones are now inactive at least in the campanian latial area seismic data indicate that orl is older than 41pl and support the interpretation that the 41pl represents a deep seated transfer fault system formed in response to the different rates of opening of the tyrrhenian sea the strike slip movements along the 41pl faults and the normal movements along the orl faults are consistent with a nw se extension which is responsible for the longitudinal extension in the southern apennines belt

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived

isotope and chemical microsampling constraints on the history of an s type rhyolite san vincenzo tuscany italy fine scale heterogeneities in strontium isotope ratios 40ar 39ar ages and chemical com sition have been determined for individual mineral grains and enclaves in the rhyolites of san vincenzo italy the rhyolites are peraluminous with phenocrysts of alkali feldspar piagioclase biotite and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry group b lavas are distinguished from group a in having higher mgo and cao contents as well as containing two pyroxenes and chilled latite enclaves the group b lavas show textural evidence for d sequilib um such as resorption and sieve zones in feldspars laser fusion 40ar 39ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4 38 0 04 ma though small amounts of excess argon are present in subhedral and cloudy crystals the groundmass in the group a sample has a higher initial r7sr 86sr 0 725 than that of the group b 0 7 13 the sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within grain zonation the feldspars and coarse grained biotites have initial 87sr 8 sr that are intermediate between those of the group a and b glasses with alkali feldspar generally having higher initial 87sr 86sr ratios than plagioclase chilled latite enclaves which are present in only the group b lavas have relatively low initial 8 sr 86sr ratios in the range 0 7082 0 7088 the mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and therefore cannot be the source of the strontium isotope dis equilibrium xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts clinopyroxene orthopyroxene clots orthopyroxene plagioclase clots and plagioclase with extensive sieve textures these grains all have relatively low initial 87sr 86sr ratios other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite spinel and sillimanite and alkali feldspars with excess 4 ar the strontium isotope di quilib um in phenocrysts of plagioclase alkali feldspar and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration a mode1 for the san vincenzo magma chamber is proposed in which the sr isotope evolution of the group b rhyolites is due to magma mixing of a crustal melt with latite magma containing unra diogenic strontium the variable initial strontium isotope compositions of group a phenocrysts may be due to assimilation or crystallization along the interface between mingled group a and b magmas

alkali calcic and alkaline post orogenic po granite magmatism petrologic constraints and geodynamic settings abstract the end of an orogenic wilson cycle corresponds to amalgamation of terranes into a pangaea and is marked by widespread magmatism dominated by granitoids the post collision event starts with magmatic processes still influenced by subducted crustal materials the dominantly calc alkaline suites show a shift from normal to high k to very high k associations source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle affected by dehydration melting and generating more and more k and lile rich magmas in the vicinity of intra crustal magma chambers anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves the post collision event ends with emplacement of bimodal post orogenic po suites along transcurrent fault zones two suites are defined i the alkali calcic monzonite monzogranite syenogranite alkali feldspar granite association characterised by biotite plagioclase fractionation and moderate lile hfse enrichments and ii the alkaline monzonite syenite alkali feldspar granite association characterised by amphibole alkali feldspar fractionation and displaying two evolutionary trends one peralkaline with sodic mafic mineralogy and higher enrichments in hfse than in lile and the other aluminous biotite bearing marked by hfse depletion relative to lile due to accessory mineral precipitation alkali calcic and alkaline suites differ essentially in the amounts of water present within intra crustal magma chambers promoting crystallisation of various mineral assemblages the ultimate enriched and not depleted mantle source is identical for the two po suites the more primitive lile and hfse rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere present rock compositions are a mixture of major mantle contribution and various crustal components carried by f rich aqueous fluids circulating within convective cells created around magma chambers in favourable areas po suites pre date a new orogenic wilson cycle

on the post 25 ma geodynamic evolution of the western mediterranean during the neogene and quaternary western mediterranean geodynamics were dominated by the eastward migration of the apenninic arc and associated back arc basins the migration was controlled by retreat of the apenninic slab and was associated with boudinage of the lithosphere in the back arc area palaeo reconstruction of the kinematics of the arc suggests about 775 km of migration from the late oligocene to present along a transect from the gulf of lions to calabria a maximum of 135 km of n s converge occurred between africa and europe during the same time span the western mediterranean was thus mainly shaped by the migration of the slab related to west directed subduction it is hypothesized that minor n s convergence deformed the arc but was not the cause of its formation 1998 elsevier science b v all rights reserved