magma mixing in boninite sequence of chichijima bonin islands abstract boninite from chichijima can be divided into two major types a porphyritic type phenocryst mode exceeds 1 and b aphyric type phenocryst mode is less than 1 both types are further subdivided by means of microphenocryst assemblages into five subtypes i olivine ol clinoenstatite cen bronzite brz ii ol brz iii brz iv brz pigeonite pig augite aug v cen brz there is a systematic correlation between these rock types and the bulk composition of aphyric boninites the most magnesian boninites are of type i and the least magnesian ones are of types iii and v type ii is slightly higher in cao than type i at a given feo mgo ratio type iv is distinct from the others for its high cao and low incompatible element contents at a given feo mgo ratio type iv cannot be derived from the others by crystallization differentiation compositional data for the other types of boninite define rectilinear compositional variations which lie on a tie line connecting fe rich bronzite and primitive boninite microphenocrysts in equilibrium with boninite magma are more magnesian so it would appear that the fe rich bronzite was in equilibrium not with boninite magma but with some evolved descendant e g bronzite andesite magma the variations of aphyric boninites are then best explained by a combination of mixing of primitive boninite and evolved descendants together with equilibrium crystallization processes in order to account for this combination of processes a density stratified open system magma chamber of the type proposed for oceanic ridge systems seems appropriate the upper part of the magma chamber is composed of differentiated bronzite andesite magma and the lower part consists of hot dense and primitive boninite magma crystallization and sporadic overturn and mixing give rise to the spectrum of magma types erupted at the surface

along axis magmatic system in the northern oman ophiolite implications of compositional variation of the sheeted dike complex 1 along axis variations for 70 km from the northern propagating tip at wadi fizh to the southern margin of the hilti block in the northern oman ophiolite are studied on the basis of bulk rock and pyroxene compositions of the sheeted dike complex less evolved dikes with tight compositions predominate at the segment center wadi thuqbah ays area while dikes with much broader compositions occur at both northern and southern ends the appearance of highly evolved rocks at the segment margin suggests more intensive fractional crystallization at the margin paradoxically primitive rocks occur at both ends indicating processes other than simple crystallization these features are interpreted as follows a more steady state melt lens at the segment center would result in less evolved melts with tight compositions and a smaller melt lens at the segment margin due to cooler conditions would result in two melts evolved melts ascribed to a more closed system fractional crystallization and primitive melts emplaced from depths without stagnancy in the melt lens zr nb and y nb ratios of the bulk analyses are constant over the whole segment indicating a comparatively uniform source along the segment for quantitative considerations 26 analyses were selected from 125 analyses na8 0 and ti8 0 values are lower at the segment center than at the north and south parts partial melting degrees are estimated to be about 20 at the center and 16 at the segment boundary respectively this is consistent with clinopyroxene compositions of the sheeted dike complex and layered gabbros the pyroxenes are lower in na2o at the segment center than those at the segment margin as compared with the same mg suggesting independently a higher degree of partial melting at the segment center the trace element contents of the sheeted dike complex show morb signatures however the ti8 0 and si8 0 of the sheeted dike complex are lower and slightly higher than modern n morbs respectively this implies that the source mantle beneath paleo ridges which produced the oman ophiolite was different from those of modern mid ocean ridges

thick sections of layered ultramafic cumulates in the oman ophiolite revealed by an airborne hyperspectral survey petrogenesis and relationship to mantle diapirism abstract using the hymap instrument we have acquired visible and near infrared hyperspectral data over the maqsad area of the oman ophiolite 15 60 km this survey allowed us to identify and map the distribution of clinopyroxene rich cumulates inter layered clinopyroxenites and wehrlites whose occurrence was previously undocumented in this area the cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the maqsad former spreading centre they occur either in mantle harzburgites as km sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene rich pyroxenite and gabbronorites or at the base of the crustal section where they are conformably overlain by cumulate gabbros these ultramafic cumulates crystallized from silica and mg rich melts derived from a refractory mantle source e g high cr low al 2 o 3 low tio 2 these melts are close to high ca boninites although strictly speaking not perfect equivalents of present day supra subduction zone boninites chemical stratigraphy reveals cycles of replenishment mixing and fractional crystallization from primitive high mg melts typical of open magma chambers and migration of inter cumulus melts the tio 2 content of clinopyroxene is always low 0 2 wt but quite variable compared to the associated pegmatites that are all derived from a source ultra depleted in high field strength elements hfse this variability is not caused by fractional crystallization alone and is best explained by hybridization between the ultra depleted melts parent melts of the pegmatites and the less depleted mid ocean ridge basalts morb parent of the dunitic troctolitic gabbroic cumulates making up the crustal section above the maqsad diapir we propose that following a period of magma starved spreading the maqsad mantle diapir impregnated with tholeiitic melts of morb affinity reached shallow depths beneath the ocean ridge this diapir induced melting of the formerly accreted and hydrothermally altered lithosphere at this stage these boninitic like lithospheric melts crystallized as pegmatitic dykes as the diapir continued to rise the amount of morb reaching shallow depths increased together with the surrounding temperature leading to the formation of magma chambers where the crystallization of layered cumulates became possible these cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of morb derived melts was moderate relative to the lithospheric derived melts as the contribution of morb to the refilling of the magma chamber increased gabbroic cumulates started to crystallize

a new magma chamber model based on structural studies in the oman ophiolite abstract structural mapping in the mafic ultramafic transition zone and in the overlying crustal section of the oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites the crustal section near the spreading axis and in particular the magma chamber are fed by two independent melts a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow whatever its origin the layering orientation in the newly created crust does not necessarily reflect its original attitude the final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen a new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges

mantle crust transition zone and origin of wehrlitic magmas evidence from the oman ophiolite abstract abundant exposure and well preserved outcrops in the oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment the mantle crust transition zone shows large lateral variations in thickness and composition but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations and gabbro lenses compositionally the transition zone marks a gradational passage from mantle to crust the moho may be placed at the base of the continuous gabbroic crustal section structures both within the transition zone and near the base of the crust are nearly parallel to the moho however the dominant flow mechanisms differ from the upper mantle to the lower crust plastic flow dominates in the upper mantle while viscous flow resulting in strong magmatic fabrics dominates at crustal levels wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted the parent magma of the wehrlitic instrusions is in fact a crystal melt mixture composed of olivine and chromite probably including a component of mantle xenocrysts and xenoliths and of reequilibrated basaltic melt

comment on dating the geologic history of oman s semail ophiolite insights from u pb geochronology by c j warren r r parrish d j waters and m p searle

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

synchronous seafloor spreading and subduction at the paleo convergent margin of semail and arabia the semail ophiolite is the largest and best exposed ophiolite having preserved intact record of a precollision system related to neotethyan ocean closure breaking with the alternative models of ophiolite exhumation and emplacement this paper builds on integrating the joined contribution of tectonic evolution of the continental margin in the premise of the ophiolite obduction connecting the along strike segmentation of the ophiolite recorded in the lithospheric flow structure and recent high precision dating results in a 3 d model of evolution of the ophiolite and its continental margin during jurassic cretaceous time among the relevant characteristics newly accounted 1 the contamination of the ophiolite by fluids and magmas of continental origin 2 the discrimination of different types of metamorphics associated to the ophiolite and conventionally classified metamorphic soles 3 the instability of the arabian margin during the late jurassic cretaceous times 4 the new precise ages relating to overlapping of spreading and detachment and 5 the inferred vergence of neotethyan closure during late mesozoic are stressed the complex stem revealed may relate to processes associated with spreading center subduction at ridge trench transform rtt triple junction as documented along a convergent margin the semail ophiolite emplacement on the arabia continental margin would result from a temporary arabian platform directed subduction zone that consumed the southern tethyan plate during late jurassic cretaceous time

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

microseismicity of the mid atlantic ridge at 7 s 8 15 s and at the logatchev massif oceanic core complex at 14 40 n 14 50 n lithospheric formation at slow spreading rates is heterogeneous with multiple modalities favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate we report microseismicity from three deployments of seismic networks at the mid atlantic ridge mar two networks surveyed the mar near 7 degrees s in the vicinity of the ascension transform fault three inside corner high settings were investigated however they remained seismically largely inactive and major seismic activity occurred along the center of the median valley in contrast at the logatchev massif core complex at 14 degrees 45n seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted logatchev hydrothermal vent field to the north and south of the massif however seismic activity occurred along the ridge axis emphasizing the asymmetry of seismicity at the logatchev segment focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3 5 km depth which we interpret to reflect volume expansion accompanying serpentinization at shallower depth of 2 4 km some earthquakes in the vicinity of the vent field showed normal faulting behavior suggesting that normal faults facilitates hydrothermal circulation feeding the vent field further a second set of cross cutting faults occurred indicating that the surface location of the field is controlled by local fault systems

osmium isotope systematics of the proterozoic and phanerozoic ophiolitic chromitites in situ ion probe analysis of primary os rich pgm in situ 187 os 188 os ratios are determined on os rich platinum group minerals in podiform chromitites both in the proterozoic ophiolite eastern desert egypt and in the phanerozoic oman ophiolite because they have very low re os these primary minerals reflect the initial 187 os 188 os ratios of their parental magmas the platinum group minerals pgm in the central eastern desert chromitites exhibit sub chondritic to chondritic 187 os 188 os ratios 0 1226 on average which is lower than the primitive upper mantle evolution trend of a comparable age those of the southern eastern desert chromitites have more radiogenic os with supra chondritic 187 os 188 os ratio of about 0 1293 on average which could be due to crustal contamination the three chromitite types in the northern part of the oman ophiolite are almost indistinguishable in terms of their 187 os 188 os ratios they have overlapping values ranging from sub chondritic to supra chondritic ratios the pge rich mantle chromitite samples have a wide range of 187 os 188 os ratio from 0 1230 up to 0 1376 with an average of 0 1299 the values of the pge poor mantle chromitites overlap in their 187 os 188 os ratios with pge rich chromites but are less variable and have a significantly higher average ratio the moho transition zone mtz chromitites are highly variable in the 187 os 188 os ratio ranging from 0 1208 up to 0 1459 the wide range of 187 os 188 os ratios from 0 1192 to 0 1459 in platinum group minerals in egyptian and oman ophiolites can be attributed to the diversity of origin of their podiform chromitites the os isotope data combined with spinel chemistry indicate that the way involved in podiform chromitite formation was not substantially different between the proterozoic ophiolite of egypt and the phanerozoic ophiolite in northern oman the os isotope compositions of the mantle chromitites in the proterozoic ophiolite of egypt clearly suggest crustal contamination the heterogeneity of 187 os 188 os ratios combined with the spinel chemistry and high pge contents of the pge rich chromitite in the oman ophiolite may give reliable evidence for high degree partial melting at a supra subduction zone setting crustal contamination from the subducted slab and assimilation of previously altered lower crustal gabbro may have contributed to the high cr spinel and radiogenic os characteristics in chromitite formed in the mantle section and along the moho transition zone respectively

constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the oman ophiolite we present major and trace element as well as sr nd and hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the oman ophiolite the rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt sio2 i e from gabbros to tonalites the samples are grouped into a ti rich and relatively light rare earth element lree enriched p1 group ce yb n 0 7 resembling the early v1 lavas and a ti poor and lree depleted p2 group ce yb n 0 7 resembling the late stage v2 lavas based on the geochemical differences and in agreement with previous structural and petrographic models we define phase 1 p1 and phase 2 p2 plutonic rocks felsic magmas in both groups formed by extensive fractional crystallization of olivine clinopyroxene plagioclase apatite and ti magnetite from mafic melts the incompatible element compositions of p1 rocks overlap with those from mid ocean ridges but have higher ba nb and th nb trending towards the p2 rock compositions and indicating an influence of a subducting slab the p2 rocks formed from a more depleted mantle source but show a more pronounced slab signature these rocks also occur in the southern blocks with the exception of the tayin block of the oman ophiolite implying that the entire ophiolite formed above a subducting slab initial nd and hf isotope compositions suggest an indian morb type mantle source for the oman ophiolite magmas isotope compositions and high th nb in some p2 rocks indicate mixing of a melt from subducted sediment into this mantle

mercury anomalies in upper aptian lower albian sediments from the tethys realm abstract the upper aptian lower albian stratigraphic interval documents important perturbations in the carbon cycle associated with the oceanic anoxic event 1b oae 1b tentatively connected with pulses in volcanic activity and the formation of the southern kerguelen plateau however uncertainties in existing age models for this interval preclude a robust and definitely convincing cause effect connection between these two events a sedimentary record from the poggio le guaine section umbria marche basin shows anomalously high levels of mercury during the upper aptian lower albian interval the persistence of the anomalies of hg after normalization against toc al and fe suggests that chemical and physical processes related to organic matter scavenging adsorption onto clay minerals hydrous iron oxides and sulphur only played a secondary role as mechanisms for hg accumulation in the sediments in agreement with previous studies in which anomalous enrichments of hg in sedimentary records were interpreted as monitors of enhanced volcanic activity we speculate that the hg enrichments documented in the poggio le guaine section reflect a rapid increase in atmospheric hg content associated with a massive release of volcanic hg and possibly suggest a multi phase emplacement of the southern kerguelen plateau skp as a realistic source nonetheless comparison of hg profiles and micropalaeontological data from the poggio le guaine section reasonably excludes the possibility that the global biotic turnover horizon documented in the upper aptian could be directly attributed to the volcanic pulses of the skp emplacement

the death of an accretion zone as evidenced by the magmatic history of the sumail ophiolite oman abstract the ophiolitic crustal sequence of the salahi block in the northern part of oman shows a complex magmatic history the oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates isotropic gabbros a sheeted dike complex and a first tholeiitic lava series v1 depleted in compatible elements the second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite v2 directly linked as can be seen from field evidence to more differentiated high level intrusive plutons this volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements a third volcanic event v3 exhibiting a transitional character enriched in lree s caps the ophiolite these v3 lavas are separated from the v2 unit by a 15 m thick pelagic sedimentary section a geotectonic evolution of the sumail ophiolite is proposed on the basis of field mineralogical and geochemical data after the oceanic accretion phase that gave birth to the main ophiolitic sequence a second stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode such a model accounts well for the depleted character of the v2 lavas the compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second stage melts which are then emplaced in the crust and partly extruded the v3 volcanism postdates the intraoceanic detachment of the ophiolite but precedes its obduction onto the arabian continental margin it is interpreted as an intraplate seamount type magmatism

the subduction initiation rule a key for linking ophiolites intra oceanic forearcs and subduction initiation we establish the subduction initiation rule sir which predicts that most ophiolites form during subduction initiation si and that the diagnostic magmatic chemostratigraphic progression for sir ophiolites is from less to more hfse depleted and lile enriched composi tions this chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during si the magmatic chemostratigraphic progression for the izu bonin mariana ibm forearc and most tethyan ophiolites is specifically from morb like to arc like volcanic arc basalts or vab boninites or bon because si progressed until establishment of a mature subduction zone morb like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of si the contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early si but continued melting results in a depleted harzburgitic residue that is progressively metasomatized by fluids from the sinking slab subsequent partial melting of this residue yields typical ssz like lavas in the latter stages of si if si is arrested early e g as a result of collision morb only ophiolites might be expected consequently morb and ssz only ophiolites may represent end members of the si ophiolite spectrum the chemostratigraphic similarity of the mariana forearc with that of ophiolites that follow the sir intimates that a model linking such ophiolites oceanic forearcs and si is globally applicable

moist morb axial magmatism in the oman ophiolite the evidence against a mid ocean ridge origin the oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid oceanic ridge basalt morb its internal structure is regarded by many as being directly comparable to ocean lithosphere from the east pacific rise however there has long been controversy over the geodynamic setting in which the ophiolite formed and the extent to which the analogy can be drawn because the morb like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites to some this implies that the entire ophiolite formed above a subduction zone others maintain that it formed at a true open ocean mor and that the water required to generate the arc like magmas derived from descending near axis hydrothermal fluids or from ancient subduction a popular compromise posits that the axial suite formed at a true mor and the later magmatism documents the initiation of obduction we test these models by reexamining the morb like character of the early axial lavas and dikes we show that fractionation trends require the presence of water at concentrations significantly higher than any open ocean morb instead trends are identical to those of backarc basin and intraoceanic forearc volcanics by showing that the entire ophiolite formed in a hydrous system we rule out all models in which the oman ophiolite was generated at an open ocean mor instead it formed at a submarine spreading center above a probably newly initiated subduction zone

origin of mesozoic ophiolitic melanges in the western yarlung zangbo suture zone sw tibet abstract the petrogenesis and tectonic evolution of the mesozoic ophiolitic melanges in the western section of the yarlung zangbo suture zone yzsz remain controversial in this paper we present the results of whole rock geochemical and sr nd isotope analyses zircon u pb ages and in situ lu hf isotopic data obtained from mafic rocks of the northern and southern sub belts of the western yzsz mesozoic ophiolitic melanges to help us understand these controversial issues diabases and dolerites from the northern sub belt and gabbros from the southern sub belt exhibit variable fore arc basalt fab like geochemical compositions and have zircon u pb ages of 126 4 120 3 ma in addition gabbro diabases from the northern sub belt have boninite series affinities and yield a zircon u pb age of 125 7 ma these results along with previous studies on the yzsz mesozoic ophiolitic melanges and the gangdese arc reveal that the western yzsz mesozoic ophiolites were likely generated over multiple stages in the epicontinental gangdese fore arc basin as the yarlung zangbo neo tethyan ocean subducted northward in front of the lhasa terrane the early cretaceous fab like and boninite series mafic rocks were formed by the reinitiation of subduction which was followed by a retreat of the subduction zone and the creation of the fore arc basin and strong hyperextension accompanied by asthenosphere upwelling at 130 120 ma during this process the upwelling asthenosphere underwent decompressional melting with limited penetration of slab derived fluids and gave rise to the n morb normal mid ocean ridge basalt like basaltic magmas that intruded the overlying previously generated depleted mantle as fab like gabbro diabase and dolerite sills or dykes then boninitic magmas represented by boninitic gabbro diabases were generated by remelting the extremely depleted residual mantle source which was metasomatized by a small amount of slab derived fluids following previous extractions of fab like magma

sub seafloor epidosite alteration timing depth and stratigraphic distribution in the semail ophiolite oman abstract pervasive epidotization of igneous rocks is a common feature in the ophiolite record of hydrothermally altered oceanic crust current genetic models view epidosites as markers of focussed upflow of hydrothermal fluid beneath oceanic spreading ridges the epidosites are envisaged to form at the base of the sheeted dike complex sdc during active plate spreading our mapping of the semail ophiolite in oman has revealed abundant epidosites in the volcanic sequence some exceeding 1 km2 in extent they are more frequent and far larger than the mineralogically identical epidosites in the sdc we have also found epidosites that traverse the entire sdc from bottom to top thus rather than being restricted to the base of the sdc as implied by current models epidosites in fact occur throughout the sdc and dominantly within the overlying volcanic pile we report the occurrence of 19 epidosite bodies and their crosscutting relations with respect to host lava units dikes intrusive stocks and also seafloor umbers the volcanostratigraphic affiliation of the dikes is identified by their whole rock and clinopyroxene compositions the relations set constraints on the timing of epidotization with respect to igneous activity in the ophiolite at least one of the epidosites in the sdc formed during lasail off axis volcanism another epidosite in the sdc and many in the volcanic units formed later during post spreading alley and boninitic alley supra subduction zone volcanism only permissive not compelling evidence allows just two of the epidosites to have formed within the main stage sdc during or shortly after its emplacement we conclude that epidotization of the oceanic crust is not necessarily coupled to spreading ridges and that it can occur during fore arc volcanism this finding is consistent with evidence from the modern seafloor and it requires a different hydrothermal environment to that traditionally associated with alteration beneath spreading axes the timing and stratigraphic constraints show that some of the epidosites formed within the top 1 4 km of the crust others between 1 2 and 1 7 km depth whereas the deepest formed between 2 6 and 3 8 km below the paleo seafloor finally the timing constraints do not prescribe a strict temporal relationship between epidotization and the formation of the numerous seafloor massive sulphide vms deposits known in the ophiolite however a role for epidosites as source rocks for the metals in the deposits remains permissible

forced subduction initiation recorded in the sole and crust of the semail ophiolite of oman subduction zones are unique to earth and fundamental in its evolution yet we still know little about the causes and mechanisms of their initiation numerical models show that far field forcing may cause subduction initiation at weak pre existing structures while inferences from modern subduction zones suggest initiation through spontaneous lithospheric gravitational collapse for both endmembers the timing of subduction inception corresponds with initial lower plate burial whereas coeval or delayed extension in the upper plate are diagnostic of spontaneous or forced subduction initiation respectively in modern systems the earliest extension related upper plate rocks are found in forearcs but lower plate rocks that recorded initial burial have been subducted and are inaccessible here we investigate a fossil system the archetypal semail ophiolite of oman which exposes both lower and upper plate relics of incipient subduction stages we show with lu hf and u pb geochronology of the lower and upper plate material that initial burial of the lower plate occurred before 104 million years ago predating upper plate extension and the formation of semail oceanic crust by at least 8 myr such a time lag reveals far field forced subduction initiation and provides unequivocal direct evidence for a subduction initiation mechanism in the geological record

dynamics of intraoceanic subduction initiation 2 suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle here we focus on suprasubduction zone ssz ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame ssz ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation structural petrological geochemical and plate kinematic constraints on their kinematic evolution show that ssz crust forms at fore arc spreading centers at the expense of a mantle wedge thereby flattening the nascent slab this leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during ssz crust crystallization former spreading centers are preserved in forearcs when subduction initiates along transform faults or off ridge oceanic detachments we show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone upon inception of slab pull due to e g eclogitization the sole is separated from the slab remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge we propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust our study highlights how far field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle

compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments abstract podiform chromitites in the mantle sections of ophiolites belong to either high cr metallurgical or high al refractory varieties their highly variable compositions are reflected by different cr s 100cr cr al and cr2o3 and al2o3 contents of the chromite falling in the boninitic and morb fields respectively parental magmas of high cr chromitites have higher sc mn co and ni and lower ti v zn and ga concentrations than morb melts their trace element patterns are similar to those of boninites except for ni and zn in contrast high al chromitites have parental magmas characterized by generally flat morb normalized patterns showing slight enrichments in v mn and co and depletion in ni and zn regardless of their compositions both types of chromitites have chondrite normalized platinum group element pge patterns showing enrichment in ipge and depletion in ppge a variety of platinum group minerals are typically present in both types occurring either as euhedral inclusions or along fractures in chromite grains these minerals have a wide span of re os isotopic compositions reflecting a variety of origins there is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites the presence of these minerals suggest that grains of amphibolite plagioclase amphibole and zircon and eclogite coesite kyanite and garnet were present in the magmas from which chromite crystallized multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments ssz we propose a new model in which chromitite formation was involved in intra oceanic subduction zones initiated in closing oceanic basins continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist amphibolite and eclogite facies conditions the tearing and breakoff of the subducted slab possibly along the transitional contact between amphibolites and eclogites create a slab window through which the underlying asthenosphere rises and melts to generate cr rich mafic magmas these upward migrating magmas pass through the subduction zone and assimilate the subducted slab as a result of slab contamination these magmas become more siliceous more oxidized and more hydrous rapidly triggering chromite crystallization minute grains of chromite are suspended in the upward moving magmas as they migrate through the overlying metasomatized mantle wedge such chromite bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid oceanic ridges due to the mantle convection it is also possible that some diamonds may have formed in the subducted slab below about 150 km some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the ssz melts some of these ssz mantle wedges are emplaced on land to become podiform chromitite bearing ophiolites

basalts erupted along the tongan fore arc during subduction initiation evidence from geochronology of dredged rocks from the tonga fore arc and trench a wide variety of different rock types were dredged from the tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 boomerang voyage 40ar 39ar whole rock and u pb zircon dating suggest that these fore arc rocks were erupted episodically from the cretaceous to the pliocene 102 to 2 ma the geochemistry suggests that mor type basalts and dolerites were erupted in the cretaceous that island arc tholeiites were erupted in the eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time the ages generally become younger northward suggesting that fore arc crust was created in the south at around 48 52 ma and was extended northward between 35 and 28 ma between 9 and 15 ma and continuing to the present day the episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the bonin marianas arc the bonin marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 ma at the beginning of subduction above a rapidly foundering west dipping slab instead a model where the 52 ma basalts that are presently in a fore arc position were created in the arc back arc transition behind the 57 35 ma loyalty three kings arc and placed into a fore arc setting after arc reversal following the start of collision with new caledonia is proposed for the oldest rocks in tonga this is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time

geology geochemistry and tectonic significance of mafic ultramafic rocks of mesoproterozoic phulad ophiolite suite of south delhi fold belt nw indian shield abstract a thick sequence of mafic ultramafic rocks occurs along a major shear zone phulad lineament running across the length of aravalli mountain range for about 300 kms it has been suggested that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite mantle component followed by layered cumulus gabbroic rocks crustal component a complex of non cumulus rocks comprising hornblende schists gabbros sheeted dykes and pillowed basalts structurally overlies layered gabbros huge bodies of diorite intrude volcanics geochemical classification suggests that all non cumulus mafic rocks are sub alkaline basalts except one variety of dykes which shows mildly alkaline character the sub alkaline rocks are tholeiite to calc alkaline with boninite affinity tectono magmatic variation diagrams and morb normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks the mafic rocks of phulad ophiolite suite are zoned across the strike in terms of their distribution from west to east the hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes the alkaline dyke occurs at the easternmost part the rocks of phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction the geochemical interpretation presented in this study together with discussion of lithological association is used to decipher the tectonic evolution of the mesoproterozoics of nw indian shield

upper crustal podiform chromitite from the northern oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for cr concentration a new type of podiform chromitite was found at wadi hilti in the northern oman ophiolite it is within a late intrusive dunite body possibly derived from olivine rich crystal mush between the sheeted dike complex and upper gabbro this chromitite forms small 30 cm in thickness pods with irregular to lenticular shapes neither layering nor graded bedding is observed within the pods the chromitite is in the upper crust by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the moho transition zone to the upper mantle it is dis tributed along a small felsic to gabbroic melt pool within the dunite body which was formed by melting of gab broic blocks captured by the mush chromian spinel was precipitated due to mixing of two kinds of melt a basaltic interstitial melt from the mush and an evolved possibly felsic melt formed by the melting of gabbro blocks the podiform chromitite reported here is strik ingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions the for mer contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions chromian spinel of the upper crustal podiform chromitite from oman has relatively low content of cr2o3 a l 2o3 the cr cr al atomic ratio of around 0 6 and the relatively high tio2 content ranging from 1 to 3 wt we conclude that assimilation of relatively si rich materials crustal rocks or mantle orthopyroxene by olivine spinel saturated melts can explain the genesis of any type of chromitite

a neoarchean dismembered ophiolite complex from southern india geochemical and geochronological constraints on its suprasubduction origin abstract ophiolites the remnants of ancient oceanic lithosphere have been described from collisional sutures of various ages with only few examples from archean terranes here we report the discovery of a neoarchean ophiolite suite from the southern margin of the dharwar craton in india tectonically intercalated within a neoproterozoic suture zone the metamorphosed and variably dismembered ophiolite suite exposed around devanur comprises altered ultramafic units websterite gabbros mafic dykes amphibolites trondhjemites and pegmatites associated with ferruginous metachert structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet the major and trace element geochemistry of the mafic dykes indicate derivation from basaltic andesitic magmas with tholeiitic to calc alkaline characteristics the rocks display negative nb anomalies with enrichment of lile k rb ba th and depletion in hfse ti nb hf tb the tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting we present new shrimp zircon u pb data for two trondhjemite samples from this complex which yield 238u 206pb ages of 2528 61 and 2545 56 ma the neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area the suprasubduction zone assemblages and arc magmas suggest a neoarchean ocean closure along the southern margin of the dharwar craton

high resolution chemostratigraphy of the late aptian early albian oceanic anoxic event oae 1b from the poggio le guaine section umbria marche basin central italy an integrated study of multiple geochemical proxies 13ccarb 13corg toc hi caco3 trace elements al ratios of the late aptian early albian oceanic anoxic event oae 1b has been performed on the pelagic sedimentary sequence of poggio le guaine umbria marche basin central italy a comparison of the newly collected stable isotope carbon curve with the records from the vocontian basin se france dsdp site 545 and hole 1049c provided a reliable and precise identification of the four main prominent black shale levels 113 jacob kilian urbino paquier and leenhardt that characterize the oae 1b the studied record shows an increase in the marine organic carbon accumulation rate in particular in the 113 jacob and urbino paquier levels in the other black shales toc values are 1 with evidence of degraded marine organic matter completely anoxic conditions were never established during the sediment deposition although evidence of oxygen depletion at the bottom of the basin is clearly documented by the distribution pattern of redox sensitive trace metals the results suggest an increase in organic carbon burial rates during the oae 1b due to the effect of enhanced surface productivity as supported by a major increase in ba al and reduced bottom water ventilation notably the kilian and urbino paquier levels from the plg section are characterized by the absence of correlative shifts in 13ccarb and 13corg the increase in the 13corg values in these levels is explained by an increase in the relative contribution of 13c enriched marine planktonic archaeal biomass while the concomitant negative excursions recorded in the 13ccarb could reflect a major contribution of isotopically light terrestrial carbonate ions from increased continental runoff during documented more humid conditions

epr microplates a model for the oman ophiolite structural mapping in the southern half of the oman ophiolite has revealed a palaeoridge organization with similarities to an epr microplate forming in superfast spreading conditions a nw trending propagator was rapidly opening in a lithosphere no more than 1 myr older and itself created in a ne sw ridge system the nw trending propagator underlined by small mantle diapirs was active or dying when detached as part of a future ophiolite local thrusting of the future ophiolite was also initiated very early between 1 and 5 myr after ridge accretion it was accompanied by a 40 rotation within this time lapse such an activity supports the comparison of the oman palaeoridge system with an oceanic microplate and provides evidence to suggest the existence of thrusts at active microplates

geochemical evidence in clinopyroxenes from gabbroic sequence for two distinct magmatisms in the oman ophiolite abstract in the oman ophiolite one of the best preserved and most studied ophiolites in the world two distinct petrogenetic suites of gabbroic rocks from the layered gabbro sequence of the wadi haymiliyah section is established using trace element chemistry of ca rich clinopyroxenes the earlier gb1 suite is characterized by plagioclase with lower an ca ca na content and clinopyroxene with low large ion lithophile elements lile concentrations the later dwgb2 suite contains plagioclase with rather high an content and clinopyroxene with high lile this difference in clinopyroxene chemistry can be extended to the extrusive rocks in this section lower earlier hv1 suite with low lile clinopyroxene and upper later hv2 suite with high lile clinopyroxene difference in lile concentration of clinopyroxenes is essentially due to geochemical difference in parental magmas the gb1 hv1 suites formed at fast spreading mor setting and dwgb2 hv2 suites at ssz setting supporting a model of transition from mid oceanic ridge to supra subduction zone settings of the oman ophiolite our results indicate that geochemical signature of clinopyroxene is a very strong tool for identification of tectonic setting of ophiolites

archaean crustal growth processes in southern west greenland and the southern superior province geodynamic and magmatic constraints abstract eo to mesoarchaean greenstone belts e g 3800 3700 ma isua c 3075 ma ivisaartoq 3071 ma qussuk occur within orthogneisses of the southern west greenland craton greenstone belts are composed mainly of metavolcanic rocks with minor ultramafic and sedimentary schists compositionally volcanic rocks are dominantly tholeiitic basalts boninites and picrites with minor intermediate to felsic volcanic rocks these greenstone belts appear to have formed in convergent margin geodynamic settings detailed field observations contrasting ages and metamorphic and structural histories suggest that this craton was assembled in several accretionary tectonothermal events involving accretion of arcs back arcs forearcs and continental fragments by horizontal tectonics the superior province of canada was also built by the amalgamation of oceanic and continental fragments ranging in age from 3700 to 2650 ma during five discrete tectonothermal events over 40 ma between 2720 and 2680 ma the neoarchaean 2750 2670 ma wawa greenstone belts are composed of tectonically juxtaposed fragments of oceanic plateaux oceanic island arcs back arcs and siliciclastic trench turbidites following juxtaposition these diverse lithologies were collectively intruded by syn to post kinematic granitoids with subduction zone geochemical signatures oceanic island arc lavas are easily distinguished from oceanic plateau counterparts because they possess positively fractionated rare earth element la sm cn 1 and gd yb cn 1 and high field strength element depleted nb th pm pm c 1000 km scale subduction accretion complex that formed along an intra oceanic convergent plate margin during trenchward migration of the magmatic arc axis several first order geological observations on archaean greenstone belts of sw greenland and the superior province suggest that phanerozoic style plate tectonic models can provide an elegant explanation for their structural lithological metamorphic and geochemical characteristics

boninitic volcanism in the oman ophiolite implications for thermal condition during transition from spreading ridge to arc the discovery of boninite a typical high mgo andesite in the oman ophiolite is reported the boninites in the oman ophiolite occur as lavas and dikes of the alley volcanic sequence that overlie or crosscut the spreading ridge derived lavas geotimes volcanic sequence and sheeted dikes the phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the izu mariana forearc boninites indicating that the alley boninites represent primitive melt generated by partial melting of hydrous peridotite the occurrence of boninite provides strong thermal and chemical constraints on the formation of the oman ophiolite that require hot hydrous shallow mantle 1250 c at 30 km depth to have underlain the proto oman ophiolite at the time of boninite generation the initiation of subduction of the young hot oceanic lithosphere and obduction of the future oman ophiolite near the spreading ridge and the resultant melting of the highly depleted shallow mantle wedge metasomatized by slab derived fluid represent the most favorable mechanism for the genesis of the alley boninites

source composition and distribution of the fluid in the kurile mantle wedge constraints from across arc variations of b nb and b isotopes b and nb concentrations and b isotope composition were determined for lavas covering the entire length of the kurile arc b nb and 11b of these lavas show clear across arc variations in which both are highest at the trench side and continuously decrease as the slab depth increases b nb in the volcanic front lavas 8 25 is significantly higher than those of morb and oib indicating that the mantle wedge is extensively metasomatized by b rich fluid derived from the subducting slab the frontal 11b values are also out of the range of morb and oib and systematic difference exists between northern kurile nk 5 4 0 5 and central and southern kurile csk 4 3 0 3 however these two parameters approach or coincide with mantle values at the most back arc side the 11b and 87sr 86sr linearly correlate with nb b suggesting control by simple mixing of two isotopically homogeneous components that is slab derived fluid and mantle wedge the estimated 11b and 87sr 86sr of the fluid are 7 0 0 6 and 0 70326 0 00005 for nk and 6 3 0 3 and 0 7033 for csk respectively these fluid compositions are consistent with a source constituted by 95 altered oceanic crust and 5 oceanic sediment with the slight difference between nk and csk resulting from the different lithologies of subducting sediments the mantle wedge beneath the kurile arc has b sr and nd isotope compositions identical with morb but shows significantly higher b nb the above trends bear a close resemblance to those observed in the izu arc strongly suggesting their universality in the arc system the across arc decline of b nb observed both in kurile and izu implies that the fluid flux from the slab continuously decreases as the slab depth increases however this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore arc region in contrast to b k is uniformly enriched across the arc this requires control by a k bearing mineral phase either in the slab phengite or the mantle wedge phlogopite although such a mineral is responsible for only a small part of the whole fluid transport

recognition of cryoturbation for classifying permafrost affected soils cryoturbation is a dominant pedologic process in permafrost affected soils and is used to delineate gelisols in soil taxonomy and cryosols in the canadian and recently proposed world reference base for soil resources and cryozems in russian systems of soil classification in this paper we summarize evidence for cryoturbation that can be used for classifying soils containing permafrost based on a literature review and our own observations cryoturbation in the soil profile is manifested by irregular and broken horizons and textural bands involutions organic matter accumulation on the permafrost table oriented stones silt caps and accumulations and deformed soil material associated with movements due to ice and sand wedge growth

synchronous formation of the metamorphic sole and igneous crust of the semail ophiolite new constraints on the tectonic evolution during ophiolite formation from high precision u pb zircon geochronology abstract the semail oman united arab emirates and other tethyan type ophiolites are underlain by a sole consisting of greenschist to granulite facies metamorphic rocks as preserved remnants of the underthrust plate sole exposures can be used to better understand the formation and obduction of ophiolites early models envisioned that the metamorphic sole of the semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement however calculated pressures from granulite facies mineral assemblages in the sole suggest the metamorphic rocks formed at 35 km depth and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle up to 15 20 km we have used high precision u pb zircon dating to study the formation and evolution of the metamorphic sole at two well studied localities our previous research and new results show that the ophiolite crust formed from 96 12 95 50 ma our new dates from the sumeini and wadi tayin sole localities indicate peak metamorphism at 96 16 and 94 82 ma 0 022 to 0 035 ma respectively the dates from the sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust and were later juxtaposed with the base of the ophiolite these data combined with existing geochemical constraints are best explained by formation of the ophiolite in a supra subduction zone setting with metamorphism of the sole rocks occurring in a subducted slab the 1 3 ma difference between the wadi tayin and sumeini dates indicates that in contrast to current models the highest grade rocks at different sole localities underwent metamorphism and may have returned up the subduction channel at different times

the conversion tectonics from spreading to subduction paleostress analysis of dike swarms during the subduction initiation in the oman ophiolite

slab controlled elemental isotopic enrichments during subduction initiation magmatism and variations in forearc chemostratigraphy abstract supra subduction zone ssz ophiolites formed in forearc settings and the izu bonin mariana ibm in situ forearc show conversions from normal mid ocean ridge basalt morb to boninitic affinities which are widely interpreted as rules for the evolving chemical geodynamics of subduction initiation magmatism sim nevertheless forearc sequence generated by subduction initiation need not always include boninitic rocks here we reconstruct a forearc chemostratigraphy without boninitic affinity based on the central palawan and amnay cpa ophiolites of the west philippines to show diversity in melt evolution of sim zircon u pb dating on the morb type amnay gabbros reveal a 10 m y of seafloor spreading from 33 to 23 ma these ages together with the progressively enriched zircon in situ oxygen isotopes 18 o 1 62 10 1 and whole rock elemental and nd hf isotopic signatures la yb n 0 19 3 45 nb yb 0 07 5 24 end 2 1 10 2 ehf 11 2 24 1 indicate that the depleted morb mantle dmm type source of the amnay ophiolite was getting enriched by fluxing of aqueous slab fluid in the early stage followed by addition of adakitic and nb enriched slab melts in the later stage this distinct mantle evolution together with higher ehf at a given end of the amnay mafic samples than the contemporaneous south china sea scs oceanic samples do not support previous interpretation that the amnay ophiolite was a fragment of the scs however the 33 23 ma amnay ophiolite can be the younger part of the 34 ma forearc type central palawan ophiolite and both i e the cpa ophiolites represent on land fragments of the nw sulu sea we propose that the 34 23 ma cpa ophiolites and the nw sulu sea constitute forearc basin of the late oligocene middle miocene island arc backarc basin duo of the cagayan ridge and se sulu sea associated with subduction of the cretaceous to eocene proto scs since 34 ma the cpa ophiolites lack boninite but show geochemical progression from normal to enriched morb affinities whose melt evolution differs from that of the ibm sim we attribute the differences in sim and variations in forearc chemostratigraphy to the subduction of a younger slab with thick terrigenous sediment cover of the proto scs versus an older slab with thin pelagic sediment cover of the pacific plate

along strike variation of the sheeted dike complex in the oman ophiolite insights into subaxial ridge segment structures and the magma plumbing system 1 along strike variations of the 60 km long sheeted dike complex in the northern oman ophiolite were studied in order to understand the shallow magma plumbing system beneath the fossil fast spreading ridge the presence of numerous dikes intruding into the layered gabbro defines the northern end of the paleoridge segment at wadi fizh the postulated segment center is located at wadi thuqbah which has a thicker mono transition zone than elsewhere along the fossil segment aphyric dikes predominate in the sheeted dikes of which 99 are simple while multiple and composite dikes are few the thickness of 1511 dikes ranges from 13 m with an average thickness of 71 3 cm restored dike trends in the 30 km long northern half of the dike complex display the ns trending north domain and nne ns trending south domain bordered at the south of wadi bani umar al gharbi in the domain boundary dikes gradually change strikes or are mutually intrusive dike thickens northward with the largest peak along wadi fizh at the northern end of the paleoridge segment and a small peak at wadi bani umar al gharbi most dikes have a bulk mg of 55 66 which overlaps the majority of morb less common highly evolved dikes with mg 34 40 characterize the north domain thicker dikes 3 m tend to have high mg while thinner dikes 2 m are highly variable in mg the regional variations of the dike trends and the whole rock compositions can be explained by the secular variation in the structures of the paleoridge segment comparable to a third order segment of the present mid ocean ridge system spanning a few tens of thousands of years initially the segment long melt lens developed along the paleoridge axis which fed long and thick dikes with high mg with decreasing supply of magma the melt lens split up into the north and south smaller lenses bordered by a deval that fed thinner dikes intruding into the former thick dikes cut off of the northern melt lens from the magma source changed the melt composition to highly evolved low mg magmas which subsequently intruded as short evolved dikes meanwhile the main melt lens in the segment center continued feeding the high mg dikes which were maintained by the larger melt lens size and intermittent magma supply from deep magma chambers