multiple sources of quaternary tephra layers in the mariana trough abstract tephra layers in quaternary sediments stolper and newman 1994 model of binary mixing of source components with one endmember being the nmorb source of stolper and newman 1994 and the other one being a compositionally variable slab derived h2o rich component similar to the h2o component of stolper and newman 1994 this interpretation implies that the variation in k2o content in the arc derived glasses about a factor of 3 at 60 wt sio2 reflects a magmatic diversity that is inherent to the central mariana arc and is not necessarily reflective of temporal changes of the composition of arc magma sources

geochemistry of tertiary igneous rocks of northern luzon philippines evidence for a back arc setting for alkalic porphyry copper gold deposits and a case for slab roll back oligocene to early miocene volcanic rocks are preserved in the central cordillera range and cagayan valley of northern luzon philippines basaltic and andesitic rocks of the pugo formation in the baguio district of the central cordillera were intruded by the 27 to 20 ma calc alkaline central cordillera diorite complex in the southern cagayan valley the subalkaline to alkaline late oligocene mamparang formation overlies the cretaceous caraballo formation and has been intruded by the didipio igneous complex the cordon syenite complex and the palali batholith the didipio complex comprises an early suite of diorites which were intruded by the strongly mineralized stocks of the dinkidi cu au porphyry deposit whole rock geochemical data for intrusive and extrusive rocks of the baguio district range from low k calc alkaline to shoshonitic basalts to dacites with rare earth element ree and high field strength element hfse characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt samples from didipio display higher alkali contents but similar trace element characteristics new age dates for the didipio area range from 25 7 to 24 8 ma the potassic magmas of the cagayan valley are interpreted to have formed in a back arc coeval to the mainarc sequence that is preserved in the baguio miocene rocks this contradicts earlier models which invoke an early miocene arc reversal in the northern luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes the lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back arc magmas in the east in northern luzon is best interpreted as the result of eastward subduction since the late oligocene the presence of 20 ma adakitic magmas in the baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back arc rifting

evaporitic mixed water dolomitization on st croix u s v i abstract dolomite exists in only a small area of pliocene carbonates on st croix this area consists of reef lagoonal and platform facies that underlie and rim the shoreline of krause lagoon an embayment that existed on the southern coastline of st croix prior to industrial development in the 1960s since dolomite is not found in similar facies elsewhere on st croix this geographic distribution suggests that the formation of the dolomite was related to hydrologic conditions in krause lagoon the dolomite is calcium rich 57 60 mol ca and exists both as euhedral rhombs 2 75 m in diameter and as a replacement mineral in dolomitized bioclasts bioclasts in the dolomitic strata often show good preservation of microstructure particularly in large benthic forams and coralline algae the mean 87sr 86sr isotopic composition of the dolomite is 0 70887 0 00002 2s n 3 which corresponds to the 87sr 86sr ratio of late miocene seawater however because the dolomite resides in pliocene strata it is difficult to invoke unmodified seawater as an agent of dolomitization dolomitization therefore requires a source of nonradiogenic strontium modern st croix groundwater has 87sr 86sr compositions between 0 7076 and 0 7085 n 4 well below the ratio of both modern seawater and the dolomites mixing calculations show that modern st croix groundwater could be a significant source of nonradiogenic strontium in a dolomite formed f om a two component groundwater seawater mix on the basis of strontium concentration modeling the groundwater component responsible for the st croix dolomites may have ranged from 40 to 80 of the dolomitizing fluids stable isotopic values for the dolomite range from d18o of 0 7 to 3 8 and d13c of 0 6 to 2 4 pdb with increasing d18o and d13c values from the margins to the center of the lagoon the maximum d18o values in these dolomites are too high to have formed fro unaltered groundwater or seawater even accounting for ice volume effects therefore the isotopically heaviest dolomite must have precipitated from fluids enriched in 18o probably as a result of evaporation dolomitization from fluids produced from a mixture of evaporated seawater and st croix groundwater are consistent with the geochemistry and geologic distribution of the dolomite calculations show that such a scenario is possible and may be fairly common despite the relative complexity of the model documented block faulting of the krause lagoon area may have provided a stable hydrologic regime for a long enough time for dolomite to form despite island uplift during the late tertiary other models of dolomitization can be shown to be less likely or untenable on the basis of chemical lithologic or hydrologic criteria

effects of grand cycle cessation on the diagenesis of upper cambrian carbonate deposits in the southern appalachians u s a abstract the vertical transition from the mainly subtidal alternating shale and carbonate units or grand cycles of the conasauga group middle to upper cambrian to the peritidal dolostone of the knox group upper cambrian to lower ordovician marks a major change in the early paleozoic passive margin sedimentation of the southern appalachians the grand cycles represent an interplay between intrashelf shale basin and carbonate platform deposition the end of grand cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost conasauga group strata the maynardville formation the maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone this change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the maynardville and in conjunction with the changes in the regional facies distribution at the end of grand cycle deposition extended its influence upon the burial diagenesis of these deposits the subtidal deposits of the maynardville contain a variety of calcite cements dolomite is not abundant in these deposits where it occurs mainly as a fine crystalline ferroan phase associated with argillaceous layers and pressure dissolution features the local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits diagenesis of clay minerals and pressure dissolution of carbonates ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite the subtidal deposits also contain saddle dolomite cement associated with mississippi valley type mvt minerals which indicate the involvement of externally derived fluids during late burial diagenesis of these strata the peritidal deposits on the other hand are extensively dolomitized fine crystalline dolomite most likely represents early diagenetic replacement of tidal flat carbonate sediment coarse crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines fenestrae desiccation cracks and evaporite dissolution voids are occluded with dolomite cement which is commonly complexly zoned zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement saddle dolomite in pore centers dissolutional voids and tectonic fractures formed during late burial from warm fluids associated with the migration of mvt brines the effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal maynardville and the underlying conasauga group carbonate deposits and between the peritidal deposits of the upper maynardville and the rest of the overlying knox group the depositional environments and early diagenesis also affected the burial diagenesis the changes in regional facies distribution caused by carbonate platform progradation at the end of grand cycle deposition resulted in a decrease in the proportion of siliciclastic components this decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits the maynardville is conformably overlain by carbonate deposits of the knox group this allowed the maynardville knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the ordovician foreland sevier basin to the southeast during the conversion from a passive into a convergent continental margin the nolichucky shale separates the maynardville knox succession from the carbonate deposits of the underlying conasauga group which were in contrast affected by the migration of burial fluids from the cambrian intrashelf shale basin to the west

dolostones from grand cayman british west indies abstract pervasively dolomitized tertiary strata on grand cayman are dominated by heterogeneous dolostones that are composed of both low ca calcium dolomite lcd 55 mol caco3 homogeneous dolostones formed of either lcd or hcd are rare some dolostones contain dolomite crystals that are characterized by oscillatory zones 1 10 m thick of lcd and hcd whereas others have rims of lcd surrounding cores of hcd in some dolostones bioclasts formed of lcd are embedded in a matrix of hcd an 1 18o difference between the lcd and hcd can be attributed to the natural covariance that exists between the 18o and mol caco3 in dolomite there are no significant differences in the 87sr 86sr ratios of the lcd and hcd there is no consistent vertical or lateral pattern to the distribution of lcd and hcd in the cayman dolostones even in closely spaced sampling sites such variability is more consistent with growth of individual dolomite crystals being controlled by processes that operated around the surfaces of individual crystals rather than by large scale changes in the dolomitizing fluids this interpretation is consistent with the development of oscillatory zoning which is attributed to precipitation of different phases or minerals from the same parent fluid because of competition between interface controlled and diffusion controlled growth kinetics dolomitization of the tertiary strata on grand cayman was probably mediated by seawater or modified seawater nevertheless small scale intrinsic factors controlled the incorporation of ca in the dolomite lattice and accordingly the geochemical parameters that are intimately linked to the mol caco3 these small scale intrinsic controls must be factored into any model to explain the formation of tertiary island dolostones

forty million years of magmatic evolution in the mariana arc the tephra glass record tephra glasses retrieved from 10 deep sea drilling project dsdp cores around the mariana arc system make up a remarkable record of explosive volcanism in the marianas over the past 40 m y major element compositions for approximately 1800 tephra glasses from basalt through rhyolite are reported and used to examine the nature and history of this activity three maxima of volcanic explosivity presumably related to especially vigorous volcanism in the mariana arc are identified with the biggest maximum at around 18 11 ma and two other maxima at 35 24 and 6 0 ma the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western pacific arcs explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception no boninitic glasses were found the tephra glasses belong to the low to medium k suite except during an enigmatic phase of medium to high k explosive volcanism during the late miocene 11 7 ma even though mariana tephra glasses are largely similar in composition to mariana arc lavas the tephra show a much larger compositional range than that found for subaerial mariana arc lavas which totally lack dacitic to rhyolitic volcanic products the tephra glasses define a bimodal population in terms of silica content with a pronounced minimum or daly gap around 65 66 sio2 mariana tephra glasses are fractionated with the least evolved glass being fe rich and having a magnesium number mg 100mg mg fe 2 of 55 basaltic tephra glasses contain 16 a1203 average 14 3 a1203 and are not high alumina basalts this contrasts with the observation that modern mariana arc lavas contain 15 21 average 17 4 a1203 the high alumina basalt lavas of the mariana arc probably reflect plagioclase accumulation and not liquid compositions all tephra glasses plot near low pressure cotectics and reaction curves on the subprojection olivine clinopyroxene quartz mafic and felsic samples define distinct trends the mafic trend reflects fractional crystallization of mantle derived basaltic magma whereas the felsic trend may be due either to anatexis of mariana arc crust or to fractionation of mafic melts the tephra glass data reinforce the model that the magmatic evolution of the mariana arc has been dominated by low pressure fractionation perhaps accompanied by anatexis episodic changes in melting regime to generate miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading these episodic changes are superimposed on a long term increase in potassium that reflects progressive metasomatism of the mantle source long term increases in k20 contents for mariana arc magmas inferred from the tephra glass record are 0 004 wt m y mafic 0 011 wt m y intermediate and 0 023 wt m y felsic

origin of k2o sio2 trends in volcanoes of the mariana arc lavas from volcanoes in the mariana arc show a range of k 2 o sio 2 trends even though the volcanoes are all nearly the same distance above the benioff zone the preferred petrologic explanation for the k 2 o sio 2 variations invokes differences in the degree of partial melting required to produce lavas for each volcano from a relatively homogeneous source material the mariana arc volcanoes appear to evolve chemically in a manner similar to oceanic volcanoes e g hawaii with early and late alkalic stages dominated by an intermediate tholeiitic stage if volcanoes in other arcs also undergo this evolution many of the anomalies in k 2 o depth relations in modern arcs could be explained

origin of atoll dolomites distinguished by geochemistry and crystal chemistry kita daito jima northern philippine sea abstract kita daito jima is a carbonate island located at the northwestern region of the philippine sea 25 55 6 57 6 n 131 16 9 19 8 e dolomites extend from the island surface to a depth of 100 m below the ground surface mbgs strontium isotope stratigraphy indicates that subsurface units c1 0 49 7 mbgs and c2 49 7 103 4 mbgs were dolomitized at 5 5 ma and 2 0 ma respectively and that island surface dolomites are products of dolomitization at 1 6 2 0 ma x ray diffraction analysis indicates that the island surface and borehole dolomites comprise variable mixtures of four and three dolomite crystal phases respectively each of these phases is distinguished by a different ca and mg content three textural types can be recognized in the kita daito jima dolomites fabric preserving crystalline nonmimetic cnm crystalline mimetic cm and fabric preserving microsucrosic ms cnm dolomites contain more calcian phases whereas ms dolomites commonly are richer in more stoichiometric phases backscattered electron images indicate that calcian dolomite phases were precipitated earlier than the more stoichiometric dolomite phases and that there is no significant hiatus between the phases although they are diachronous both the island surface and borehole cores dolomites show linear relationships between whole rock 18o composition and mg contents and between whole rock trace element concentrations sr and na and mg contents these covariances result from phase mixing not stoichiometric effects deconvolution of whole rock isotopic and elemental compositions based on the relative abundance of phases reveals that each phase has a distinct chemical and isotopic composition oxygen isotopic compositions of the island surface and borehole dolomites suggest that all dolomite phases formed in seawater and that dolomitization primarily occurred during glacio eustatic sea level lowstands and cooler ocean temperatures

along arc variations in the pre eruptive h2o contents of mariana arc magmas inferred from fractionation paths new experimental results are presented on the effects of dissolved h2o during low pressure 52 kbar crystal fractionation in mariana arc magmas the resulting fractionation paths are used to infer the pre eruptive h2o concentrations of mariana lavas from seamounts and islands estimated pre eruptive h2o concentrations for mariana seamounts 2 3 wt are relatively constant and similar to three out of eight of the subaerial volcanoes alamagan pagan and asuncion these h2o concentrations are also similar to measured h2o concentrations in mariana back arc lavas lavas from the remaining islands guguan agrigan uracas anatahan and possibly sarigan have significantly higher h2o 5 6 than the other seamounts and islands these high h2o islands are distributed throughout the arc and do not show any relation to variations in subduction parameters such as subduction angle or obliquity analysis of the h2o concentrations of olivine hosted melt inclusions from seven islands guguan pagan agrigan anatahan alamagan asuncion and sarigan and one seamount fukujin match the estimates inferred from the liquid lines of descent one interpretation of our results is that the flux of volatiles from the mariana sub arc mantle is fundamentally heterogeneous alternatively the variations in pre eruptive h2o concentrations could reflect variations in the depth and extent of fractionation of mariana magmas

nd and sr isotopic compositions of lavas from the northern mariana and southern volcano arcs implications for the origin of island arc melts nd and sr isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern mariana and southern volcano arcs values of end range from 2 4 to 9 5 whereas 87sr 86sr ranges from 0 70319 to 0 70392 these vary systematically between and sometimes within arc segments the nd and sr isotopic compositions fall in the field of ocean island basalt oib and extend along the mantle array lavas from the volcano arc mariana central island province and the southern part of the northern seamount province have end to 10 and 87sr 86sr 0 7032 to 0 7039 these are often slightly displaced toward higher 87sr 86sr at similar end in contrast those lavas from the northern part of the mariana northern seamount province as far north as iwo jima show oib isotopic characteristics with end and 87sr 86sr 0 7035 to 0 7039 plots of 87sr 86sr and end versus ba la and la yb n support a model in which melts from the mariana and volcano arcs are derived by mixing of oib type mantle or melts therefrom and a metasomatized morb type mantle or melts therefrom an alternate interpretation is that anomalous trends on the plots of nd and sr isotopic composition versus incompatible element ratios found in some s nsp lavas suggest that the addition of a sedimentary component may be locally superimposed on the two component mixing of mantle end members

unzipping of the volcano arc japan abstract a working hypothesis for the recent evolution of the southern volcano arc japan is presented which calls upon a northward progressing sundering of the arc in response to a northward propagating back arc basin extensional regime this model appears to explain several localized and recent changes in the tectonic and magrnatic evolution of the volcano arc most important among these changes is the unusual composition of iwo jima volcanic rocks this contrasts with normal arc tholeiites typical of the rest of the izu volcano mariana and other primitive arcs in having alkaline tendencies high concentrations of light ree and other incompatible elements and relatively high silica contents in spite of such fractionated characteristics these lavas appear to be very early manifestations of a new volcanic and tectonic cycle in the southern volcano arc these alkaline characteristics and indications of strong regional uplift are consistent with the recent development of an early stage of inter arc basin rifting in the southern volcano arc new bathymetric data are presented in support of this model which indicate 1 1 structural elements of the mariana trough extend north to the southern volcano arc 2 2 both the mariana trough and frontal arc shoal rapidly northwards as the volcano arc is approached 3 3 rugged bathymetry associated with the rifted mariana trough is replaced just south of iwo jima by the development of a huge dome 50 75 km diameter centered around iwo jima such uplifted domes are the immediate precursors of rifts in other environments and it appears that a similar situation may now exist in the southern volcano arc the present distribution of unrifted volcano arc to the north and rifted mariana arc to the south is interpreted not as a stable tectonic configuration but as representing a tectonic snapshot of an arc in the process of being rifted to form a back arc basin

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

melt components derived from a subducted slab in late orogenic ore bearing porphyries in the gangdese copper belt southern tibetan plateau abstract a porphyry type cu mo au polymetallic ore belt extends in an e w direction for 400 km along the gangdese magma arc in the southern tibetan plateau extensive field investigations and systematic geochemical study combined with s and pb isotopic tracing indicate that cu polymetallic mineralization of the copper belt is genetically related to late orogenic granitic porphyries formed in a late himalayan crustal extension stage 18 14 ma geochemistry of the porphyries shows the essential characteristics of adakite and suggests a dominant role of subduction related components in their genesis the genesis of these ore bearing porphyries is mainly related to melt components derived from subducted slabs and has little relationship with fluids released from them pb isotopes show a clear linear array in the plumbotectonics model and are manifested by a progressive variation from orogenic pb in the east segment of the copper belt to mantle pb in the west segment forming a mixing line of indian oceanic morb and indian oceanic sediments this suggests that the porphyry magma dominantly originated from partial melting of subducted oceanic crust and was mixed with a minor melts of sediments and mantle wedge components

physical volcanology of the submarine mariana and volcano arcs narrow beam maps selected dredge samplings and surveys of the mariana and volcano arcs identify 42 submarine volcanos observed activity and sample characteristics indicate 22 of these to be active or dormant edifices in the volcano arc are larger than most of the mariana arc edifices more irregularly shaped with numerous subsidiary cones and regularly spaced at 50 70 km volcanos in the mariana arc tend to be simple cones sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110 counterclockwise from that trend suggesting a strong fault control on the distribution of arc magmas volcanos in the mariana arc are generally developed west of the frontal arc ridge on rifted frontal arc crust or new back arc basin crust volcanos in the central mariana arc are usually subaerial large 500 km3 and spaced about 50 70 km apart those in the northern and southern marianas are largely submarine closer together and generally less than 500 km3 in volume there is a shoaling of the arc basement around iwo jima accompanied by the appearance of incompatible element enriched lavas with alkalic affinities the larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos if the magma supply estimated at 10 20 km3 km of arc per million years at 18 n has been relatively constant along the mariana arc we can infer a possible evolutionary sequence for arc volcanos from small irregularly spaced edifices to large over 1000 km3 edifices spaced at 50 70 km the volcano distribution and basal depths are consistent with the hypothesis of back arc propagation into the volcano arc

shoshonitic magmas in nascent arcs new evidence from submarine volcanoes in the northern marianas volcanoes in the northern mariana arc between uracas lat 20 n and minami iwo jima 24 n are very active yet entirely submarine in contrast to the predominantly low k basaltic magmas of the central mariana arc the northern mariana arc is dominated by more siliceous melts in the south and by shoshonites in the north the northern arc melts have enrichments in ba 800 ppm rb 70 ppm sr 1000 ppm ce 50 ppm and ce yb n 24 which increase to the north as far as iwo jima lavas from volcanoes north of iwo jima lack these enrichments and are indistinguishable from those of the central maranas the shoshonites are unusual in occurring along the magmatic front of a primitive intra oceanic arc we hypothesize that they represent the reconstruction of a magmatic arc following melting of enriched mantle due to the propagation of the mariana trough spreading center northward through the volcano arc shoshonites thus may characterize the initial stages of arc construction after an episode of back arc rifting and need not be restricted to the mature stages of arc evolution this situation contrasts with subduction zone initiation where first melts may be boninites or low k tholeiites these differing initial melts converge toward tholeiitic and calc alkaline compositions as arcs evolve

ascending freshwater mesohaline mixing a new scenario for dolomitization abstract the process of mixing zone dolomitization has received enough criticism in recent years to suggest that it no longer be considered viable for making large amounts of dolomite this study proposes an alternative model that still requires fluid mixing in this model ascending flow of freshwater mixes with evaporated seawater leading to extensive dolomitization the new model is constrained by data from upper miocene strata of southeast spain where dolomite yields fluid inclusion tm ice data that range from 0 2 to 2 3 c indicating precipitation from fluids of 4 to 43 ppt seawater salt equivalent thus confirming fluid mixing the 13c and 18o of the dolomite ranges from 4 5 to 3 0 vpdb and 0 9 to 6 0 vpdb respectively and displays positive covariance although such positive covariation may represent fluid mixing the mixing of two dolomites during sampling or an originally homogeneous dolomite that has been partially altered recrystallized fluid inclusion data and cathodoluminescence petrography disprove the mixture during sampling and partial alteration hypotheses modeling of the most positive dolomite 18o values indicates an evaporated seawater endmember of 43 ppt salinity identical to that of the most saline fluid inclusions because paleotopography is preserved in the study area the spatial variation in dolomite geochemistry can be mapped to evaluate if it is consistent with the hydrogeology of a typical mixing zone model or an alternative the data reveal that both downdip and updip areas were influenced by high salinity and low salinity fluid endmembers and that stratigraphically lower units have more depleted 13c and 18o than upper units these results are consistent with a model of upward flow of freshwater that provides the mechanism of mixing of freshwater and evaporated seawater upward flow and degassing of co2 from loss of pressure contributed to nearly platform wide dolomitization in this succession and may have analogs in other ancient successions

dolomitization of the middle devonian winnipegosis carbonates in south central saskatchewan canada the middle devonian winnipegosis carbonate unit in south central saskatchewan is partially to completely dolomitized two major types of replacive dolomite are distinguished microcrystalline to finely crystalline dolomite type 1 displays nonplanar a to planar s textures mimetically replaces the precursor limestone accounts for about four fifths of dolomite phases volumetrically and mainly occurs in the winnipegosis mounds and the lower winnipegosis member directly underlying the mounds medium crystalline dolomite type 2 shows planar s to planar e textures commonly occurs in the lower winnipegosis and brightholme members and decreases upward in abundance the 87 sr 86 sr ratios of type 1 dolomite 0ae70795 to 0ae70807 fall within the estimated sr isotopic range for middle devonian marine carbonates stratigraphic petrographic and geochemical data constrain the formation of type 1 dolomite to hypersaline sea water in a near surface environment after marine cementation and sub aerial diagenesis and prior to precipitation of the middle devonian leofnard salts movement of dolomitizing fluids could be driven by density differences and elevation head the shift to lower d 18 o values of type 1 dolomite 7ae 4t o 5ae1 vienna pee dee belemnite vpdb is interpreted as the result of recrystallization at elevated temperatures during burial type 2 dolomite has higher 87 sr 86 sr ratios 0ae70809 0ae70928 suggesting that the dolomite probably formed from basinal fluids with an increased richness in the radiogenic sr isotope in type 2 dolomite sr 2 concentrations are lower and fe 2 and mn 2 concentrations are higher compared with the associated limestone and type 1 dolomite type 2 dolomite is interpreted as having been formed from upward migrating basinal fluids during latest devonian and carboniferous period

the role and impact of freshwater seawater mixing zones in the maturation of regional dolomite bodies within the proto floridan aquifer usa palaeogene dolostones from the sub surface of florida are ideal for the study of dolomite maturation because they record the early stages of a secondary dolomite overprint without destruction by later diagenetic overprints two distinct dolomite textures occur in the dolostones of the upper eocene ocala and lower oligocene suwannee limestones in west central florida a porous and permeable sucrosic dolomite and a less porous and relatively impermeable indurated non sucrosic dolomite in both textures the initial matrix dolomite is dully luminescent whereas the secondary overprint is dominantly luminescent cement in the suwannee and only neomorphic luminescent dolomite in the ocala the abundance of luminescent dolomite ranges from 2 to 38 which translates to 1 6 km3 of material in the suwannee and 13 5 km3 in the ocala extrapolated trace element contents sr and na and 18o values for the matrix and luminescent end members indicate a marine origin for the matrix dolomite in both units and a freshwater seawater mixing zone origin for the secondary luminescent dolomites the 18o values indicate that a saline middle mixing zone environment overprinted the suwannee but a more dilute mixing zone affected the ocala fluid fluid mixing models constrained by modern floridan aquifer hydrochemistry and extrapolated 87sr 86sr values of the luminescent phases indicate that the mixing zones operated during the late miocene to pliocene in the ocala and affected the suwannee in the pliocene the luminescent suwannee mixing zone cement reduced porosity up to threefold and permeability up to 100 fold which converted many sucrosic dolomites to indurated dolomites by contrast the neomorphic luminescent ocala dolomite did not have an appreciable impact on the maturations although freshwater seawater mixing zones were not the sites of the initial dolomitization the mixing zone environment did dramatically overprint and mature the regionally widespread dolomites of the ocala and suwannee limestones this maturation occurred shortly after formation of the proto floridan aquifer the timing suggests the matrix dolomites were ripe for alteration and that the only prerequisite for mixing zone dolomite is pre existing dolomite substrates to reduce kinetic barriers in contrast to recent claims the results of this study demonstrate that mixing zones can be effective in forming regionally significant amounts of secondary dolomite and influencing the petrophysical maturation of dolomite bodies

the holonda porphyrites norwegian caledonides geochemistry and tectonic setting of early mid ordovician shoshonitic volcanism petrological and geochemical data denote that the late arenig early llanvirn holonda porphyrites belong to the subduction related shoshonite association original compositions ranged from absarokite through shoshonite to latite and their very evolved nature signifies a mature arc aynity in an active continental margin setting much of the geochemical variation can be interpreted in terms of fractional crystallization followed by a varying admixture of cumulus phases to the ascending melt and accumulation of phenocrysts within the magmatic feeder system their eruptive products covered an area which was at least 20 25 km across forming local islands with carbonate rich shelves in a shallow to deeper marine environment the immediate substrate to the volcanosedimentary holonda basin is the lokken ophiolite which formed in a primitive oceanic arc marginal basin setting some 10 20 ma prior to the shoshonitic magmatic event arc continent collision led to emplacement of the oceanic arc and marginal basin crust onto the continental margin in pre late arenig times strong uplift of the obducted oceanic plate occurred in response to the buoyancy effect of the underthrusting less dense continental margin this is interpreted to have been followed by a reversal of subduction polarity and formation of a steeply dipping subduction zone against the continental margin lithosphere and was succeeded by the very evolved holonda porphyrite shoshonitic magmatism in volcanic islands along this submerged continental margin

temporal evolution of tertiary dolostones on grand cayman as determined by 87sr 86sr abstract on the cayman islands the tertiary bluff group brac formation cayman formation pedro castle formation is onlapped and overlain by the pleistocene ironshore formation on grand cayman the brac formation and cayman formation are formed of finely crystalline dolostones whereas the pedro castle formation is formed of finely crystalline dolostones dolomitized limestones and limestones no dolomite has been found in the ironshore formation dolostones in the bluff group which retained their original depositional textures and lack evidence of any recrystallization are formed of small typically 5 15 m long interlocking euhedral dolomite crystals dolomite cement is present in the brac formation and cayman formation but is very rare in the pedro castle formation most of the dolomite crystals are characterized by oscillatory zoning with alternating zones of low ca calcian dolomite and high ca calcian dolomite grand cayman is ideal for assessing the temporal evolution of tertiary dolostones because the dolostones are young have not been recrystallized and are geographically isolated by the deep oceanic waters around the island interpretation of 158 new 87sr 86sr ratios from the dolostones in the bluff group indicate that the succession underwent three time transgressive phases of dolomitization during the late miocene the late pliocene and pleistocene petrographically similar dolomite was produced during each phase of dolomitization that was mediated by the same type of fluid and the same general conditions dolomitization was part of a dynamic cycle of processes that followed major lowstands karst development during the lowstands preconditioned the limestones for dolomitization by increasing their porosity and permeability thus vast quantities of the dolomitizing fluids could freely circulate through the strata during the subsequent transgression dolomitization ceased once a stable highstand had been attained

submarine arc volcanism in the southern mariana arc as an ophiolite analogue supra subduction zone ssz ophiolites are being recognized more and more frequently in the geologic record submarine volcanism in intra oceanic arcs is one tectonic setting in which such ophiolites might form the southern seamount province of the mariana arc represents the best documented example of an active intra oceanic submarine arc which can serve as a modern analogue for some ssz ophiolites the southern seamount province comprises nine submarine volcanoes at least seven of which are active or dormant these edifices have erupted plagioclase clinopyroxene olivine phyric basalts andesites dacites and dacitic pumices these lavas are similar to those of the subaerial edifices in the central marianas sio2 49 70 tio2 0 4 1 3 mgo 1 2 5 9 ba 140 450 ppm 87sr 86sr 0 70346 0 70354 but are somewhat more enriched in the light rare earth elements the geochemical characteristics of subaerial mariana arc lavas are preserved in the submarine lavas and the ssp lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices we are ignorant of the composition of any lavas erupted between the principal edifices which may be expected to dominate the volcanic section of ssz ophiolites nevertheless the characteristic high ratio lil hfsc low ti ni and cr and diagnostic mineral compositions calcic plagioclase relatively fe rich olivine are likely to be preserved in unaltered submarine arc lavas the mariana active arc is built on back arc or thinned frontal arc crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic hypabyssal and volcanic rocks of back arc frontal arc and active arc origin such complicated intrusive relationships if preserved in structurally disrupted ophiolites will make it difficult to reconstruct the geologic history of an ssz ophiolite

dolomite formation in caribbean islands driven by plate tectonics abstract postdepositional dolomite occurs in the shallow subsurface of several caribbean islands in tertiary and pleistocene carbonates e g on barbados grand cayman jamaica and st croix recent investigations have interpreted these dolomites as relatively early diagenetic formations from chemically modified seawater mainly on the basis of petrography stratigraphic association and stable isotope values interpretations of the hydrologic regimes during dolomitization vary and usually include freshwater seawater mixing thermal or density driven convection of seawater or a combination thereof this paper offers two alternative interpretations based on the relatively low sr isotope ratios of the dolomites 0 7085 to 0 7092 and their proximity to active plate margins dolomitization may have been driven by plate tectonics i e either by thermal convection of pore fluids at active spreading centers or by tectonic expulsion of fluids from accretionary prisms the dolomitizing solution was seawater that was enriched in 86sr by magmatic hydrothermal solutions or by interaction with siliciclastics in an accretionary prism respectively

multistage dolomitization in late jurassic early cretaceous platform carbonates berdiga formation basoba yayla trabzon ne turkey implications of the generation of magmatic arc on dolomitization abstract late jurassic early cretaceous berdiga formation which is widely distributed in an e w trending belt in the eastern pontides ne turkey is composed of platform carbonates the formation shows varying lithofacies changing from supratidal to platform margin reef both laterally and vertically the studied basoba yayla trabzon ne turkey area comprises one of the typical exposures of formation in the northern zone of eastern pontides in this area the lower parts of the formation are pervasively dolomitized by fabric destructive and fabric preserving replacive dolomites that are ca rich and nonstoichiometric ca56 60mg40 44 replacement dolomites rd are represented by 18o values of 15 3 to 4 2 vpdb 13c values of 3 7 1 5 vpdb and 87sr 86sr ratios of 0 70675 0 70731 petrographic and geochemical evidence implies that rd dolomites are formed before compaction at shallow moderate burial depths from late jurassic early cretaceous seawater and or partly modified seawater as a consequence of rock water interaction and they are recrystallized at elevated temperatures during subsequent burial during subsequent late cretaceous diagenetic process when the region became a magmatic arc rd1a and rd3 dolomite were recrystallized by hydrothermal fluids of marine origin the dolomite cements cd overlying the low amplitude stylolites have similar geochemical charasteristics than rd1a and rd3 dolomite and are characterized by high homogenization temperatures th this might indicate that cd dolomite was precipitated from hydrothermal fluids of marine origin that recrystallized the rd1a and rd3 dolomite the fact that cd dolomite comprising the latest phase of dolomitization is overlain by coarsely crystalline mosaic calcite and silica cements may indicate that diagenetic fluids were undersaturated with respect to dolomite and oversaturated with respect to calcite and silica

petrology chemistry and isotopic composition of submarine volcanoes in the southern mariana arc four submarine volcanoes in the southern and central mariana arc were dredge sampled the recovered lavas were analyzed for major and trace element abundances including rare earth elements ree mineral compositions and sr and nd isotopic ratios the samples exhibit a great range of chemical and mineral compositions relative to subaerial exposures in the central arc zealandia bank produces high magnesium basalts mgo 8 9 wt enriched in ni and with forsteritic olivine as much as fo 88 this lava composition is probably parental to more fractionated basalts and basaltic andesites exposed on the adjacent islands of guguan and sarigan the southernmost seamounts in the mariana arc south of lat 15 n produce dacitic pumice a composition that is not common in the active arc ree data preclude their derivation from a basaltic parent by fractional crystallization they may be derived by direct partial mantle melting under water saturated conditions at low pressure nd and sr isotopic data 143 nd 144 nd 0 5130 0 5131 87 sr 86 sr 0 7032 0 7035 indicate a mantle source for the mariana arc lavas similar in isotopic characteristics to sources for ocean islands significant contributions to these arc magmas from subducted oceanic crust or sediment are not evident a spatial distribution in magmatic characteristics coincides with a structural break in the arc near 16 n south of this submarine seamounts produce incompatible element enriched lavas with relatively high 87 sr 86 sr 0 7034 0 7035 these lavas were produced by a relatively low degree of partial melting 5 15 of a shallow spinel bearing mantle to the north zealandia bank and the central arc islands are characterized by lower incompatible element abundances and lower 87 sr 86 sr 0 7032 0 7033 these lavas were produced by higher degrees of partial melting 15 25 of a deeper garnet bearing mantle

geological evidence bearing on the miocene to recent structural evolution of the new hebrides arc abstract the new hebrides archipelago is a complex reversed arc system that can be divided into four major volcanic provinces the western belt is an early to middle miocene extinct volcanic arc that as a result of polarity reversal is now incorporated into the frontal arc of the present day configuration the eastern belt initially received detritus in the early middle miocene from a tholeiitic arc complex but in the mio pliocene became the locus of a more calc alkaline arc volcanism volcanic activity then ceased in the eastern belt but is well represented as a third and largely submerged marginal province through the pliocene into the early pleistocene the present volcanic line the central chain is essentially a continuation of the marginal province volcanism into recent times initial tectonic events in the new hebrides arc were associated with the regional disruption in the middle miocene of an east facing system with consequent termination of western belt arc volcanism the western belt remained as a landmass during the lowermost late miocene but subsided following a late miocene renewal of island arc volcanism to the east this latest phase was coeval with initial expansion of the north fiji basin and marked the advent of the new hebrides as a westwards migrating reversed arc system during arc migration there were apparent hiatuses in island arc volcanism the most notable being a middle pliocene to late pleistocene period of quiescence in the central sector tectonism in the early pleistocene recent raised the fore arc brought about rifting and extension to the rear and concentrated volcanism along the presently active central chain

maturation of neogene dolomite from xuande atoll of xisha archipelago the south china sea abstract dolomitization is a remarkable diagenetic process occurring in the subsurface carbonate beds at xuande atoll of xisha islands in the south china sea dolomite crystals initially appear at the depth of 180 m in holocene pleistocene carbonate with relatively small concentrations whereas extensive dolomitization has developed at the minimum burial depth of 290 m in pliocene strata of the zk 1 well petrographically zk 1 dolomites have undergone from shallow to deep depth a progressive maturation process indicated by fabric evolvement of dolomitized rocks rock lithification induced by textural coarsening and continued growth of cements increase in stoichiometry and cation ordering of dolomite downcore and alteration of metastable dolomite to stable one involving preferential dissolution and subsequent reprecipitation process under cathodoluminescence dolomite cements are mostly dull red to nonluminescent or zoned whereas the replacive matrix dolomite exhibits a homogeneous greyish red cathodoluminescence whole rock elemental analyses show that dolomite rock is characterized by low concentrations of mn

petrography of finely crystalline cenozoic dolostones as revealed by backscatter electron imaging case study of the cayman formation miocene grand cayman british west indies abstract finely crystalline cenozoic island dolostones like those found in the cayman formation on grand cayman are commonly assumed to be petrographically and compositionally homogeneous backscatter electron images bsei however show that the constituent dolomite crystals 3 content moreover such images allow 1 depiction of growth patterns in the constituent crystals irrespective of their origin 2 recognition of replacive dolomite as opposed to dolomite cement and 3 delineation of stratigraphic packages in the dolomite cements that reflect different episodes of cementation integration of this information forms the basis for paragenetic interpretations of the dolostones on grand cayman the miocene cayman formation can be divided into friable high porosity dolostones and hard low porosity dolostones backscatter electron images show that the hard dolostones are characterized by complex arrays of zoned dolomite cements that have occluded most of the pores caymanite an internal sediment has occluded many of the larger cavities in contrast the high porosity dolostones contain little cement and no internal sediments precipitation of the cements and the deposition of internal sediments were related to the passage of large volumes of water through some of the dolostones thus the hard low porosity dolostones are found in the cap rock of the formation in coastal locations and in areas close to solution widened fractures conversely the friable high porosity dolostones form the lower porous unit of the formation in the interior of the island where the passage of water was more restricted

origin of island dolostones a case study from the cayman formation miocene cayman brac british west indies abstract cayman brac the easternmost of the cayman islands is 19 km long 1 5 to 3 km wide and rises 40 m above sea level at its eastern end geographically isolated by the deep oceanic waters of the caribbean sea this island with its thick succession of pervasively dolomitized tertiary rocks is a natural laboratory for assessing the origin of island dolostones dolostones in the miocene cayman formation 100 m thick with most crystals 55 mol caco3 geochemically they are characterised by 13c values of 1 6 to 3 5 18o values of 2 3 4 0 80 279 ppm sr 52 340 ppm fe and 9 82 ppm mn the inverse correlation between 18o and mol caco3 and positive correlation between sr and mol caco3 largely reflect kinetic effects on oxygen isotope fractionation and the sr partition behaviour between dolomite and water dolomite stoichiometry indicates formation from waters with a high mg ca ratio and 18o values corrected for biases associated with dolomite stoichiometry and phosphoric acid fractionation indicate formation from normal seawater like fluids under near surface conditions the 87sr 86sr ratios point to two phases of dolomitization phase i in the late miocene 6 8 ma caused partial dolomitization of the basal part of the cayman formation phase ii during the pliocene to early pleistocene 1 5 ma completed dolomitization of the cayman formation both phases of dolomitization were mediated by similar fluids although available information suggests that dolomitization was probably linked to sea level fluctuations the timing of dolomitization relative to the transgressive regressive cycle remains open to debate

anhydrite cements after dolomitization of shallow marine silurian carbonates of the gascoyne platform southern carnarvon basin western australia abstract carbonates and evaporites in the dirk hartog group were deposited in subtidal peritidal and shallow marine evaporitic mudflat environments across the gascoyne platform within the southern carnarvon basin western australia the carbonates are composed of ooids peloids and minor bioclastic fragments with early cements they have been extensively dolomitized and replaced in part by late anhydrite sparry cements four types of dolomite which range from early to burial types are identified incorporating re equilibration or re crystallization from basin brines accordingly they exhibit distinctive petrographic features and isotopic signatures of carbon and oxygen evaporites deposited are present as discrete beds and displacive nodules in carbonate and siliciclastic beds and show 34scdt values and 87sr 86sr ratios indicative of variable marine and non marine conditions late dissolution of evaporites has produced satin spar gypsum veins in the shallowest section of the platform whereas blocky and sparry anhydrite cements and void fillings formed deeper within the platform dissolution of the evaporites and formation of anhydrite cements post date dolomitization

powder x ray diffraction analysis of homogeneous and heterogeneous sedimentary dolostones abstract the ca molar ca ca mg 100 in sedimentary dolomite which ranges from 48 ca to 62 5 ca can be determined by atomic absorption spectrometry electron microprobe analysis or powder x ray diffraction analysis the latter procedure which relies on the fact that the position of the d104 reflection depends linearly on the ca content offers an easy and relatively inexpensive method of determining ca in dolomite to date such analyses have assumed that the dolostone is homogeneous with respect to the ca content of its constituent dolomite crystals herein the xrd technique is extended to compositionally heterogeneous dolostones a considerable amount of important information can be derived from x ray diffractograms of dolostones if they are analyzed with peak fitting techniques pf xrd specifically this technique allows 1 determination of the ca with an accuracy of 0 5 ca 2 identification of heterogeneous dolostone samples formed of more than one population of dolomite crystals as defined by their ca content and 3 derivation of the proportion of each population of dolomite crystals with an accuracy of 10 the pf xrd technique is a powerful analytical tool that provides valuable information on the major element composition of low fe dolomite compared to other techniques the pf xrd technique is accurate rapid relatively inexpensive and easy to use

diffuse co2 efflux from iwojima volcano izu ogasawara arc japan abstract iwojima volcano located on the southernmost part of the izu ogasawara arc is characterized by the extrusion of trachyte or trachy andesite lavas and pyroclastic rocks of holocene and surface thermal manifestations small phreatic explosions have been recorded frequently during the last 100 years with the most recent in 1999 and 2001 in order to elucidate the behavior of volcanic volatiles and to assess the potential activity of this volcano diffuse co 2 efflux co 2 content and 13 c co 2 in soil gas and soil temperature at 30 cm depth were measured at 272 sites in march 2000 112 sites in december 2000 and 40 sites in december 2001 we found that high co 2 efflux values of more than 100 g m 2 day 1 occurred at several locations on motoyama volcano corresponding with high soil temperatures more than 60 c at 30 cm depth region and with areas where co 2 with magmatic 13 c was observed here the magmatic 13 c determined for fumarolic co 2 data ranged from 2 to 3 which is clearly higher than magmatic gas values 8 to 2 typically found in island arc settings around the world however this can be explained in terms of carbon isotope fractionation between calcite and co 2 under subsurface temperature and pressure conditions at iwojima a total efflux of co 2 for iwojima volcano is estimated to be 760 t day 1 with a magmatic contribution of about 450 t day 1 this value is rather high compared with other volcanoes in island arc settings since iwojima has no visible plume almost all volcanic co 2 is released as diffuse efflux through the volcanic edifice

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

island dolostones genesis by time transgressive or event dolomitization abstract thick cenozoic dolostone successions found on many isolated carbonate islands have commonly been attributed to dolomitization events whereby time restricted dolomitization processes repeatedly affected the original limestone successions any model invoked to explain dolomitization of these successions depends on accurate dating of the events because that may allow correlation with other processes such as sea level oscillations or climate change the 87sr 86sr ratios available from island dolostones throughout the world which have been used to date dolomitization under the premise that seawater mediated its formation indicate that most of these island dolostones developed between the late miocene and late pleistocene 10 to

stick slip motion of an antarctic ice stream the effects of viscoelasticity stick slip behavior is a distinguishing characteristic of the flow of whillans ice stream siple coast antarctica distinct from stick slip on northern hemisphere glaciers which is generally attributed to supraglacial melt the behavior is thought be controlled by basal processes and by tidally induced stress however the connection between stick slip behavior and flow of the ice stream on long time scales if any is not clear to address this question we develop a new ice flow model capable of reproducing stick slip cycles similar to ones observed on the whillans ice plain the model treats ice as a viscoelastic material and emulates the weakening and healing that are suggested to take place at the ice till interface the model results suggest the long term ice stream flow that controls ice discharge to surrounding oceans is somewhat insensitive to certain aspects of stick slip behavior such as velocity magnitude during the slip phase and factors that regulate it e g elastic modulus furthermore it is found that factors controlling purely viscous flow such as temperature influence stick slip contribution to long term flow in much the same way additionally we show that viscous ice deformation traditionally disregarded in analysis of stick slip behavior has a strong effect on the timing of slip events and therefore should not be ignored in efforts to deduce bed properties from stick slip observations

massive dolomitization of a late miocene carbonate platform a case of mixed evaporative brines with meteoric water nijar spain late miocene platform carbonates from nijar spain have been extensively dolomitized limestones are present in the most landward parts of the platform in stratigraphically lower units and topographically highest outcrops suggesting that dolomitizing fluids were derived from the adjacent nijar basin the dolomite crystals range from 10 to 100 m existing as both replacements and cements na cl and so4 concentrations in the dolomites range from 200 to 1700 p p m 250 650 p p m and 600 7000 p p m respectively comparable with other tertiary and modern brine dolomite values and also overlapping values from mixing zone dolomites sr concentrations range between 50 and 300 p p m and the molar sr ca ratios of dolomitizing fluids are estimated to range between 7 seawater brine to freshwater ratios the 18o and 13c of the dolomites range from 1 0 to 4 2 pdb and 4 0 to 2 0 pdb respectively 87sr 86sr values 0 70899 0 70928 of the dolomites range from late miocene seawater to values greater than modern seawater mixtures of freshwater with seawater and evaporative brines probably precipitated the nijar dolomites modelled covariations of molar sr ca vs 18o and na ca vs 18o from these mixtures are consistent with those of the proposed nijar dolomitizing fluids complete or partial dolomite recrystallization is ruled out by well preserved cl zoning nonstoichiometry and quantitative water rock interaction modelling of covariations of na vs sr and 18o vs 13c the possibility of multiple dolomitization events induced by evaporative brines seawater and freshwater respectively is consistent with mineral mineral mixing modelling the basin derived dolomitizing brines probably mixed with freshwater in the nijar basin or mixed with fresh groundwater in the platform and were genetically related either to deposition of the yesares gypsum or the feos gypsum dolomitization occurred during either the middle messinian or the early upper messinian nijar dolomitization models may be applicable to dolomitization of other late miocene platform carbonates of the western mediterranean moreover the nijar models may offer an analogue for more ancient evaporite absent platform carbonates fringing evaporite basins

dolomitization of the pedro castle formation pliocene cayman brac british west indies abstract the pedro castle formation pliocene on cayman brac is variably dolomitized by texture preserving but non mimetic and texture destructive replacive dolomite mimetic replacement of skeletal grains is limited to echinoderm plates and with few rare exceptions there is no mimetic replacement of red algae foraminifera green algae or any other type of skeletal grain the lack of mimetic dolomite is atypical of island dolostones found in the caribbean sea and the pacific ocean dolostones in the pedro castle formation are formed entirely of high ca calcian dolomite average of 57 4 mol caco 3 oxygen isotopes mean 1 25 pdb from the dolomite indicate that dolomitization was mediated by seawater or modified seawater carbon isotopes in the dolomite which range from 1 81 to 1 42 pdb were probably inherited from the precursor limestone the average sr content in the dolomite 360 ppm is higher than that found in most other island dolomites the sediments that now form the pedro castle formation were deposited in shallow water on an open bank during the early pliocene pre dolomitization diagenesis of those sediments included syntaxial overgrowths around echinoderm fragments dissolution of aragonitic bioclasts stabilization to low magnesium calcite and local precipitation of vadose cements thus the limestones had been extensively stabilized by the time that dolomitization took place during the late pliocene the general paucity of mimetic replacement in these dolostones can probably be attributed to the calcite stabilization that took place before dolomitization

evolution and development of miocene island dolostones on xisha islands south china sea on xisha islands located in the south china sea the neogene succession includes the unconformity bounded huangliu formation that is 210 5 m thick in well ck 2 and formed almost entirely of dolostones the diverse biota in the huangliu formation which includes corals algae bivalves and foraminifera indicates that the original carbonate sediments accumulated in water that was 30 m deep the dolostones are formed of various mixtures of low and high calcium calcian dolomite with limpid dolomite lining the walls of many cavities the o 18 and c 13 stable isotopes suggest that dolomitization was mediated by slightly modified seawater the sr 87 sr 86 ratios from the dolostones suggest that dolomitization took place similar to 9 4 and 2 3 ma ago with the age of dolomitization becoming progressively younger towards the top of the formation island dolostones like these found on many islands throughout the pacific ocean and the caribbean sea have commonly been linked to eustatic changes in sea level with dolomitization taking place during lowstands highstands or transgressive phases data from the huangliu formation in well ck 2 suggests that dolomitization was associated with semi continuous transgressive conditions that were controlled by the interaction of tectonic subsidence and eustatic changes in sea level

dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes upper muschelkalk switzerland the trigonodus dolomit is the dolomitized portion of the homoclinal ramp sediments of the middle triassic upper muschelkalk in the se central european basin various dolomitizing mechanisms followed by recrystallization have been previously invoked to explain the low 18o high 87sr 86sr extensive spatial distribution and early nature of the replacive matrix dolomites this study re evaluates the origin timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the trigonodus dolomit at 11 boreholes in northern switzerland in each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth crystal size increase change from anhedral to euhedral textures uv fluorescence decrease 18ovpdb decrease from 1 at the top to 6 7 at the base and an 87sr 86sr increase from 0 7080 at the top to 0 7117 at the base thus dolomites at the top of the unit record isotopic values similar to middle triassic sea water 18ovsmow 0 87sr 86sr 0 70775 while dolomites at the base record values similar to meteoric groundwaters from the nearby vindelician high 18ovsmow 4 87sr 86sr 0 712 according to water rock interaction modelling a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends instead the combined isotopic geochemical and petrographic data can be explained by dolomitization via seepage reflux of hypersaline brines into dense horizontally advecting groundwaters that already had negative 18o and high 87sr 86sr values evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization this study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre existing groundwaters can be preserved during seepage reflux dolomitization in low angle carbonate ramps

cathodoluminescence in quaternary carbonate deposits abstract the cathodoluminescent oscillatory and sectoral growth zones common in crystals formed in ancient limestone successions in a variety of putative environments appear to be rare or absent from recent and pleistocene marine carbonate sequences the factors controlling cathodoluminescence and reasons for this disparity are examined the cathodoluminescent zones in the cements of ancient rocks have been interpreted as responses to variations in the redox potential of formative pore waters during crystal growth although similar cathodoluminescent behaviour is recorded from some deposits including travertines and quaternary speleothems formed in what are thought to have been strongly oxidizing environments the apparent absence of cathodoluminescence in the most recent and pleistocene marine deposits that presumably reflect deposition and diagenesis in environments that are also characteristically oxidized therefore seems anomalous the controlling influences on cathodoluminescence are reviewed together with evidence relating to observations of pleistocene marine deposits and likely conditions of formation but where it is present the mechanism s for its development remain elusive

are arc type rocks the products of magma crystallisation observations from a simple oceanic arc volcano raoul island kermadec arc sw pacific raoul island is the emergent summit of a large intra oceanic strato volcano in what is globally one of the simplest of subduction settings in this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high mg numbers and high ni and cr expected of primitive mantle derived melts the lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene olivine orthopyroxene and spinel phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition indicating a decoupling of melt and crystal components in the system a consistent model for the raoul magmatic system is that primitive high mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low mg magmas by fractional crystallisation within the lower crust low mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone these processes are essentially those that characterise continental subduction related magmatic systems but differ only in the absence of an evolved crustal component

implications of paleomagnetic data on the collision related bending of island arcs paleomagnetic studies from the marianas central philippines ryukyus sulawesi fiji new hebrides all show major differences in the declination values observed from individual islands within the same arc comparable differences are not observed in the inclination values a simple plate tectonic model would predict that each island arc should experience the same rotation along its entire length as the plate moves around its rotation axis bathymetry from each of these arcs show that the differentially rotated segments of the upper plate occur in areas where the downgoing plate carries a feature such as a seamount chain rifted continental fragment or an island arc the coincidence of the bathymetric feature and the discordant paleomagnetic declination data support an earlier hypothesis of vogt 1973 that these features are buoyant zones on the downgoing plate which interact with the margin of the upper plate as a result of this interaction the upper plate boundary is locally deformed in addition to the paleomagnetic evidence other geologic and geophysical data from these areas suggest such interactions for example there appears to be a good correlation between the deflections in major structures around the collision zone and the paleomagnetic directions that are reported as this deformation proceeds eventually the stresses resulting from the collision may result in any of the following tectonic processes 1 slip line style strike slip faults allowing for the sideward extrusion of portions of the upper plate tapponnier et al 1983 2 changes in the polarity of the subduction zone 3 development of strike slip faults around the margin of the indenter or 4 reorganization of the entire plate margin in addition data from the marianas southern end of the ryukyu arc and the lau basin suggest that back arc basin development may result from the collision related deformation

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

inside out dolomite abstract inside out dolomite crystals are unusual because their cores are younger than their encasing cortices in the cayman formation middle miocene of grand cayman they developed diagenetically from zoned dolomite crystals that have cores formed of high ca calcian dolomite hcd 55 mol caco3 and outer cortical zones formed of low ca calcian dolomite lcd 3 or oscillatory zoned lcd hcd the cores of these crystals labeled as type i have a euhedral rhombic outline with no embayments or angular reentrants and commonly contain calcite inclusions and or micropores hollow dolomite crystals evolved from these crystals as preferential dissolution removed all or part of the hcd core later calcite or dolomite cement was precipitated inside the hollow crystals the former produced dolomite crystals with a calcitic core i e dedolomite whereas the latter produced inside out dolomite crystals that have a dolomite core lcd or hcd encased by a dolomite cortex the cores of the inside out dolomite crystals labeled as type ii have an irregular outline characterized by angular reentrants and or embayments micropores that are usually larger than those in the type i cores but few calcite inclusions the cortical zones have a euhedral rhombic motif irrespective of the type of core development of insideout dolomite crystals may significantly alter the average ca of the dolostones as hcd is lost to dissolution and lcd or hcd cement is precipitated in its place the 18o may also change inasmuch as it is thought to covary with ca in some tertiary dolostones the genesis of inside out dolomite crystal may offer an alternative explanation for the textures e g clotted cathodoluminescence patterns in some ancient dolomite crystals that have been attributed to replacement and or recrystallization

shoshonitic volcanism in the northern mariana arc 1 mineralogic and major and trace element characteristics petrographic and geochemical characteristics of samples dredged from 23 submarine volcanic edifices in the northern mariana and southern volcano arcs define two distinct rock series the central and northern mariana arc to 23 n and the northern volcano arc north of iwo jima are characterized by plagioclase clinopyroxene orthopyroxene titantomagnetite bearing subalkaline rocks including both low k and medium k series the northern mariana arc and southern volcano arc from 23 n to iwo jima are erupting rocks of a shoshonitic series with phenocrysts of plagioclase clinopyroxene olivine and biotite these rocks are less saturated than those of the subalkaline provinces and are substantially enriched in ba 400 900 ppm sr 600 1000 ppm k2o 1 4 5 and k2o na2o 0 4 1 2 relative to the subalkaline lavas ba y and ba zr increase by a factor of 3 to 4 in the shoshonitic rocks but k rb ba sr and k ba are relatively constant throughout the arc there is no relationship between degree of enrichment and volcano volume or degree of fractionation seamounts and islands within each province have the same range of compositions much of the intra edifice variation in lava composition can be modeled by 10 70 crystallization of plagioclase and clinopyroxene with smaller amounts of olivine orthopyroxene and titanomagnetite or by accumulation of 10 40 phenocrysts dominantly plagioclase the differences between parental lavas in the alkalic and subalkalic provinces require melting of two distinct mantle sources the occurrence of this enriched mantle in the northern marianas may be a consequence of the propagation of the mariana trough spreading center into the volcano arc

esmeralda bank geochemistry of an active submarine volcano in the mariana island arc esmeralda bank is the southernmost active volcano in the izu volcano mariana arc this submarine volcano is one of the most active vents in the western pacific it has a total volume of about 27 km3 rising to within 30 m of sea level two dredge hauls from esmeralda recovered fresh nearly aphyric vesicular basalts and basaltic andesites and minor basaltic vitrophyre these samples reflect uniform yet unusual major and trace element chemistries mean abundances of tio2 1 3 and feo 12 6 are higher and cao 9 2 and al2o3 15 1 are lower than rocks of similar silica content from other active mariana arc volcanoes mean incompatible element ratios k rb 488 and k ba 29 of esmeralda rocks are indistinguishable from those of other mariana arc volcanoes on a ti zr plot esmeralda samples plot in the field of oceanic basalts while other mariana arc volcanic rocks plot in the field for island arcs

hf nd isotope variation in mariana trough basalts the importance of ambient mantle in the interpretation of subduction zone magmas in the study of geochemical mass balances at subduction zones the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter deconvolving the influence of ancient versus modern enrichments is particularly difficult especially when considering elements that are highly mobile here we provide an alternative approach using less mobile elements and a filter to remove the effects of recent slab additions we provide new hf isotope data for 30 mariana trough mt backarc basin lavas once filtered hf and nd isotope ratios are highly correlated of indian mid oceanic ridge basalt character and display variations similar to ocean ridges of comparable lengths the isotopic variability observed in this ambient mantle provides a new paradigm for the interpretation of the varied volcanic products of the arc thus shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source in contrast the large volcanoes of the central island province have a consistent offset in nd isotope compositions from the mt array resulting from fluid addition existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale we suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide

petrology and geochemistry of the island of sarigan in the mariana arc calc alkaline volcanism in an oceanic setting isotopic studies of rocks from oceanic island arcs such as the marianas indicate that little if any recycling of continental material e g oceanic sediments occurs in these arcs because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs an important question is whether the andesites of continental arcs are produced by a fundamentally different more complex mechanism than the lavas of oceanic arcs an excellent opportunity for study of this question is provided by the island of sarigan in the mariana active arc on which calc alkaline andesites including hornblende bearing types are exposed along with more mafic lavas

depositional environments and facies of the late triassic abu ruweis formation jordan abstract the abu ruweis formation is composed of carbonates evaporites and mudstones with some locally developed pelletic oolitic and stromatolitic limestones the lateral persistence of bedding the purity of the evaporite rocks the alternating arrangement of marine carbonates and evaporites indicates periodic deposition in subaqueous conditions salina petrographic investigations x ray diffraction analysis as well as chemical analysis have shown that the outcropping evaporite beds are mainly composed of secondary gypsum with rare anhydrite relics five microfacies of gypsum were recognized according to their fabrics porphyroblastic and granoblastic gypsum showing polarization texture gypsum pseudomorph after anhydrite laths and satin spar gypsum the textures they display indicate a hydration origin of precursor anhydrite which is in turn rehydrated from primary gypsum some of these anhydrites were formed as a result of replacement processes of the carbonate sediments associated with the evaporites as evidenced from the textural relationships of the carbonate and sulfate minerals the o18 content ranges from 1 45 to 8 38 pdb and the c13 content ranges from 1 52 to 4 73 pdb trace elements analysis has shown that the abu ruweis dolomites are rich in strontium up to 600 ppm and sodium up to 835 ppm the isotope composition and trace elements content as well as the petrographic characteristics point to a penecontemporaneous hypersaline dolomitization origin for the abu ruweis dolomites the evaporites were deposited during a regressive lowstand systems tract whereas the carbonates were deposited under shallow water marine conditions during a highstand systems tract the abu ruweis succession represents a relatively stable arid climate within a rapidly subsiding basin restricted conditions were provided by the development of beach barriers

phase relations of a high mg basalt from the aleutian island arc implications for primary island arc basalts and high al basalts many volcanic centers in the aleutian islands have erupted lavas that range in composition from high mg basalt mgo 9 wt to more fractionated and voluminous high al basalts and basaltic andesites the petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated the phase relations of a typical high mg basalt from the makushin volcanic field on unalaska island provide important constraints on petrogenetic models results of one atmosphere and moderate to high pressure 5 20 kb anhydrous experiments are similar to results obtained from primitive morb at low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize glasses in equilibrium with olivine and clinopyroxene at intermediate pressure 5 to 10 kb are similar in composition to high al basalt plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly although our studies do not confirm the primary nature of high mg basalts they do support a model in which high al basalts are generated by moderate amounts of crystal fractionation from more primitive high mg mg fe lower al2o3 basaltic magmas near the arc crust mantle boundary

seismotectonics around the mariana trough abstract strike slip type mechanism solutions with approximately east west striking tension axes were obtained for large shallow earthquakes under the central topographic high of the northern mariana trough these solutions are consistent with the model that spreading is in progress in the direction perpendicular to the structural lineations trending n s in most of the trough and nnw sse in the northern part weak coupling at the subduction zone between the pacific plate and the landward plate mariana platelet is suggested by the occurrence of few thrust type and many normal fault type earthquakes near the mariana trench spreading striking nearly e w with an approximate total rate of 6 cm yr in the widest section of the trough is speculatively inferred from the plate motion model of the philippine sea plate with respect to the surrounding plates and from the anchored slab model and the collision effect of the caroline ridge with the philippine sea plate

genesis of fabric destructive dolostones a case study of the brac formation oligocene cayman brac british west indies abstract the origin of fabric destructive and fabric retentive textures in dolostones of all ages remains open to debate evolution of the fabric destructive textures which effectively destroys all traces of the original limestones remains particularly controversial on cayman brac a small island located in the caribbean sea the brac formation lower oligocene is formed of limestones and fabric destructive dolostones the top of this formation is defined by the brac unconformity which developed as a karst surface during the late oligocene and early miocene the overlying cayman formation lower to middle miocene is pervasively dolomitized and characterized by fabric retentive textures that have preserved the depositional textures of the original limestones the stratigraphic juxtaposition of these two formations provides an ideal opportunity to compare the factors that controlled the development of fabric destructive and fabric retentive fabrics in dolostones the dolostones in the brac formation are formed of dolomite that replaced the limestone and dolomite cements typically zoned that were precipitated in pores throughout the dolostones interpretations based on the composition of the dolomite 13 c values 18 o values and sr fe and mn contents indicate that dolomitization of both the brac formation and the cayman formation was probably controlled by similar slightly modified seawater with the distribution of the dolomite in the limestones and the evolution of dolostone textures being controlled largely by permeability pathways that governed circulation patterns of the dolomitizing fluids the large sucrosic dolomite crystals which are commonly zoned probably developed as a result of repeated cycles of limestone matrix dissolution and dolomite precipitation

rifting history of the northern mariana trough seamarc ii and seismic reflection surveys seamarc ii sidescan sonar data and seismic reflection profiles of the northernmost portion of the mariana trough suggest a rifting rather than a seafloor spreading origin for lithosphere within this active backarc basin of the western pacific variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin the northern mariana trough widens from 70 km near 23 n to 135 km near 21 30 n major active fault zones bound both sides of the backarc basin active rifting and volcanism are predominant in a roughly 20 km wide active rift zone along the eastern arc side of the basin from 24 n to 22 n the locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness which is greatest at the volcanic front the zone of rifting intersects the active volcanic arc at nikko seamount the distinction between the mechanism for formation of the mariana trough proposed here and that of slow mid ocean ridge spreading is that no true plate boundary has developed in the northernmost mariana trough rather the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas volcanism both in the rift zone and throughout the basin is fault controlled south of 22 n the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached the transition to true mid ocean ridge spreading occurs to the south of the survey area described here

geology and geochemistry of early arc volcanic rocks from guam the island of guam is located at the southern end of the mariana fore arc and was the site of early arc volcanism between 43 and 32 m y ago two volcanic units were erupted during the early evolution of guam the late middle eocene facpi formation and the late eocene to early oligocene alutom formation the facpi formation is composed largely of interbedded boninite series pillow lavas pillow breccias and dikes although arc tholeiite series rocks cap the formation in some areas the alutom formation is composed of interbedded volcanic breccias tuffaceous sandstones lava flows and sills calc alkaline arc tholeiite and boninite series compositions are all found in the alutom formation primitive boninite series rocks are relatively high in sio 2 mgo and ni and low in a1 2 o 3 and tio 2 contents compared to basaltic rocks from the arc tholeiite series additionally they have low ti zr ratios and high k zr rb zr and ba zr ratios although no calc alkaline basalts are found in the early arc formations of guam basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc alkaline series among the analyzed samples here are a few with anomalously high y and rare earth element ree contents chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase clinopyroxene and orthopyroxene the arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase clinopyroxene orthopyroxene and magnetite the composition of the least silicic calc alkaline series andesite can be modeled by fractional crystallization of olivine clinopyroxene plagioclase and magnetite from a transitional basalt derivation of more silicic calc alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende all basalts from the early volcanic series of guam were erupted in an arc setting the boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels whereas the arc tholeiite and calc alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels we suggest that because the calc alkaline series parent basalts most likely were richer in incompatible high field strength hfs elements and sio 2 than were the tholeiitic parents they were generated by lower degrees of melting

geochemical complexities preserved in the volcanic rocks of the zambales ophiolite philippines the geochemical characteristics of the volcanic rocks from the zambales ophiolite luzon philippines indicate that the oceanic crust now preserved in the ophiolite must have been generated in an oceanic subduction zone environment the volcanic rocks and dikes range in composition from magnesian basalt including mid ocean ridge basalt morb island arc tholeiite and boninite like lavas to andesite and dacite and have trace element abundances which suggest derivation from a mantle source which was heterogeneous on both regional and local scales the sources are mixtures of components variably depleted with respect to a morb source differences exist between respective exposures of the ophiolite from north to south the following geochemical patterns in the basaltic rocks emerge sm nd decreases with corresponding decreases in 143nd 144nd rare earth element ree patterns range from extremely light ree lree depleted with generally low abundances of all ree 1 5x chondritic to patterns which are slightly lree depleted similar to morb patterns throughout the range sr isotopic abundances are relatively high 87sr 86sr 0 7035 0 7040 perhaps more significant than the regional geochemical trends however is that all volcanic provinces display ranges of rock compositions and the spatial distribution of basalts with such variable geochemical affinities indicate superposition interfingering and possible mixing of different lavas we suggest that the array of geochemical data from the zambales ophiolite can be explained in terms of processes observed in present day convergent plate margins such as the marianas or lau basin in the western pacific complicated plate boundaries which have existed for long periods of time including closely opposing and changing subduction systems the rifting of arcs and the formation of backarc basins may result in the superposition of one lava type on another or may produce many small domains in the upper mantle sources for subduction related lavas some of which become extremely depleted or secondarily enriched magmas derived from such a heterogeneous mantle will display ranges in geochemical characteristics possibly similar to those observed in the zambales ophiolite indeed recent studies from present day backarc systems suggest that young arc volcanism is superimposed on morb like crust and that basalt compositions of certain backarc basins are zoned inward from arc like to morb like these studies indicate that relationships between arc and morb like crust in young subduction complexes is extremely complex hawkins and melchior 1985 volpe et al 1987 1990 we extrapolate arguments from studies of zoned backarc systems to the zambales ophiolite and in turn use data from the zambales ophiolite as a ruler for defining the scale of variation in these complex supra subduction zone systems

noble metal abundances of late archean 2 7 ga accretion related shoshonitic lamprophyres superior province canada abstract shoshonitic magmatism occurred in transpressive geodynamic settings within the superior province during the diachronous accretion of allochthonous subprovinces over 2720 to 2680 ma which culminated in stabilization of this archean craton the dikes are interpreted to have formed from low degrees of partial melting of depleted mantle metasomatically enriched in lile and lree during a prior subduction accretion event selenium contents of the dikes range from 50 130 ppb and s from 150 1000 ppm with s se ratios of 1150 18 000 signifying open system behaviour of s during autometasomatic and late hydrothermal alteration based on the se contents and assuming magmatic s se ratios of 5 000 primary s contents of the alkaline magmas were undersaturated at 650 ppm mgo and ni are correlated and ir during evolution of the shoshonitic magmas olivine pyroxene controlled ir concentrations and probably rh platinum and palladium correlate with ti implying that these pge were controlled by timagnetite fractionation

spatial variations in the stoichiometry and geochemistry of miocene dolomite from grand cayman implications for the origin of island dolostone abstract the cayman formation miocene 140 m thick on grand cayman is incompletely dolomitized with the most of the dolomite restricted to the peripheral part of the island these calcium rich dolomites with 50 60 mol caco 3 ca are divided into low calcium dolomite lcd ca 55 despite the small size of the island 6 8 km wide the percentages of lcd and hcd the ca and the geochemical properties of the dolomites show geographic variations relative to the surrounding shelf edge accordingly the cayman formation on the east end of the island is divided into the peripheral dolostone shelf edge 1 5 km inland transitional dolostone 1 5 2 7 km inland and the interior limestone and dolostone 2 7 km to island center from the peripheral dolostone to the interior limestone and dolostone there is an increase in the percentage of hcd and ca in the dolomite and decreases in the 18 o and 13 c values of the dolomite interpretations based on the oxygen and carbon isotopic compositions indicate that seawater is the source of mg for dolomitization the concentric pattern of dolomitization on the island reflects the fact that seawater flowed into the island from all directions during dolomitization the lateral inland variations in the dolomite stoichiometric and isotopic properties reflect the gradual modification of seawater by water rock interaction along the flow path from the shelf edge to the island center 87 sr 86 sr ratios indicate that two phases of dolomitization late miocene early pliocene and late pliocene early pleistocene were responsible for dolomitization of the cayman formation it is very likely that during both phases the carbonate platform was subaerially exposed and that the pump for circulating the seawater through the island was related to the seawater freshwater mixing zone the dolomitization model developed from grand cayman may be applicable to many other island dolostones affected by long term sea level changes given that the diagenetic potential of dolostone is directly linked to its stoichiometry the distribution of the lcd hcd will influence the petrographic properties geochemical signatures and reservoir potential of the dolostones