the role of magma mixing in the petrogenesis of mafic alkaline lavas rockeskyllerkopf volcanic complex west eifel germany the quaternary rockeskyllerkopf volcanic complex rvc comprises three spatially and temporally distinct volcanic centers that can also be distinguished on the basis of their geochemical signatures all the volcanic products in the complex are olivine basanites whose major and trace element compositions span almost the entire range defined for the west eifel field as a whole the rvc lavas have lower al2o3 na2o and y contents and higher tio2 cao k2o sc v co rb and ba than the tertiary lavas in nearby hocheifel volcanic field within the complex the oldest south east lammersdorf center sel comprises primitive lavas with an average mgo content of 11 wt and lan ybn of 29 2 the second center mauseberg has similar mgo to sel but is distinct in its much higher lan ybn of 42 2 the rockeskyllerkopf center which was erupted after a break in activity comprises lavas similar in composition to the sel center but with distinctly higher al2o3 and lower mgo contents given the lack of evidence for significant fractionation or assimilation in the rvc lavas we attribute the compositional variations within and between the centers of the rvc to be due to variations in the composition of the source region in combination with magma mixing our preferred model involves 1 5 partial melting of lree enriched mantle in the garnet stability field likely within the thermal boundary layer at the base of the lithospheric mantle these melts mixed to variable degrees with 2 4 partial melts of phlogopite spinel peridotite formed at higher levels in the modally metasomatised lithospheric mantle

assimilation and crystal accumulation in a mid crustal magma chamber the sausfjellet pluton north central norway the sausfjellet pluton is made up of two intrusive units emplaced into high grade metamorphic rocks of the helgeland nappe complex of the uppermost allochthon in the norwegian caledonides the eastern part of the pluton intruded marble and less voluminous calc silicate and pelitic rocks the western half is hosted predominantly by semi pelitic migmatite with intercalated marble remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide the early gabbroic unit forms the southeastern part of the body it consists of hornblende bearing to hornblende rich gabbro and diorite which is thought to have crystallized from an h2o rich andesitic parental magma the younger dioritic unit underlies the central and western parts of the pluton as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks herein the annular zone the interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite the western and annular zones are by comparison mineralogically heterogeneous they range from diorite to quartz monzonite and from biotite bearing two and threepyroxene assemblages to biotite hornblende assemblages neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact stoped blocks of a distinctive coarse grained diorite as well as pyroxenerich calc silicates are present in the gabbroic unit and the central zone of the dioritic unit the few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz rich gneiss chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes plagioclase from an h2o poor andesitic parent these rocks have approximately constant d 18 o of 6 6f0 2x and lack evidence of in situ assimilation heterogeneities in the western and annular zones of the dioritic unit are reflected in variable anomalously enriched incompatible element contents and in d 18 o which ranges from 6 7x to 8 6x petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones if so evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks as much as 20 of the mass of the western and annular zones can be ascribed to assimilated material which apparently entered the magma by stoping therefore the asymmetrical

crustal contamination identified in keweenawan osler group tholeiites ontario a trace element perspective in the lake superior area voluminous middle proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the midcontinent rift and overlie archean and older proterozoic basement the 3000 m thick section of gently dipping 10 sse basalts on the black bay peninsula is divided into three suites based on the relationship between stratigraphy petrography and geochemical variations e g mg number a1 2 o 3 la sm and gd yb the lower suite consists of augite olivine porphyritic lavas with high mg number sio 2 tio 2 and gd yb but low la sm the overlying central suite flows are either aphyric or plagioclase porphyritic and have lower mg number sio 2 la sm and gd yb decline upward through the central suite the upper suite lavas are either aphyric or plagioclase porphyritic and element ratios plot on slightly different trends compared to the central suite the lower suite has a narrow compositional range the average of the four most primitive lavas is used

structure and evolution of the rockeskyllerkopf volcanic complex west eifel volcanic field germany the rockeskyllerkopf volcanic complex rvc comprises three overlapping monogenetic volcanic centers southeast lammersdorf sel mauseberg m and rockeskyllerkopf rkk each volcanic center comprises proximal wall deposits with a well defined crater wall unconformity and crater fill deposits that partially to completely cover the outer crater wall the sel center is a phreatomagmatic tuff ring composed of lithic rich tephra deposited by pyroclastic falls and surges the second center mauseberg with its crater to the northwest of the sel center is predominantly magmatic topographic and outcrop patterns suggest that this center may have formed a series of overlapping scoria cones along a n s trending fissure the youngest center rkk which lies on a poorly developed palaeosol within the earlier mauseberg deposits comprises a well developed proximal crater wall sequence this sequence of magmatic likely strombolian fall and grain avalanche deposits passes upward into a crater fill sequence that comprises variably welded bombs the final eruptions in the center were massive lava flows that were ponded within the rkk crater ar ar age dating of reequilibrated fragments of phlogopite megacrysts in the sel lavas indicates volcanic activity began at 474 39 ka literature k ar dates for the youngest lava flows in the rkk center give ages of 360 60 to 470 ka our interpretation of the age data and the presence of the poorly developed palaeosol between the mauseberg and rkk centers indicates that volcanism in the rvc began around 470 ka with the eruption of the sel and mauseberg centers followed a few thousand years later by the eruption of the rkk center

assimilation of crustal xenoliths in a basaltic magma chamber sr and nd isotopic constraints from the hasvik layered intrusion norway contaminated layered intrusions known thousands of recrystallized strontium and neodymium isotopic data for mafic cumulates chilled tabular xenoliths of metasedimentary origin enclosed in the cumulates margins and adjacent crustal rocks of the hasvik layered intrusion are thought to represent the remnants of the assimilated material north norwegian caledonides are reported together with new the xenoliths spalled off the roof during magma emplacement and mineralogical and whole rock analytical data to constrain the extent together with the elevated temperatures 400 600 c of the midand effect of the assimilation of crustal xenoliths in a basaltic crustal country rocks led to a high degree of assimilation in the magma chamber initial sr sr 700 ma of 0 7045 and end hasvik magma chamber 700 ma of 3 03 for the chilled margin which has a tholeiitic composition akin to the chilled rocks of the skaergaard intrusion demonstrate that the parental magma was derived from a depleted mantle source the basal cumulates 0 335 m show an up section decrease in sr sr from 0 7045 to 0 7038 and a correlative

the temporal evolution of three magmatic systems in the west eifel volcanic field germany abstract the quaternary west eifel volcanic field is the surface expression of a mantle plume that is thought to contain 1 melt the volcanoes range in age from 0 7 ma to 10 000 yr bp and the average recurrence rate of volcanism is around 3 5 10 4 eruptions per year giving a mean time between eruptions of 2875 years assuming that the field is monogenetic the presence of mafic and ultramafic xenoliths is taken as evidence for rapid single stage emplacement of magmas at velocities around 0 5 km h however the presence of clinopyroxene that preserves evidence of polybaric crystallisation belies this simple model this study uses variations in the fo content of olivine in mantle derived xenoliths and in olivine xenocrysts enclosed in cumulate xenoliths to calculate the time scale of evolution of three volcanic centres in the eifel field core rim variations in olivine are interpreted to result from fe mg interdiffusion between it and a surrounding melt using experimental determinations of the interdiffusion coefficient and approximations of magma temperature contact times have been calculated using planar and radial diffusion models the three volcanic centres studied were meerfelder maar gemundener maar and baarley the cumulate xenoliths at meerfelder maar formed at a depth of 13 km and record a magma emplacement event at between 1 and 3 years prior to eruption two suites of mantle derived olivine clinopyroxenites record a further two events at 1 8 days and 3 h prior to eruption the final event is interpreted to record the eruption of this volcano and the 3 h contact time records the time required for transport of peridotite xenoliths from the mantle to surface giving an average emplacement velocity of around 15 km h the cumulate xenoliths at gemundener maar crystallised at depths of between 4 and 22 km there are four distinct events recorded by the zoned olivine grains in the clinopyroxene rich cumulates from this locality the maximum residence time is similar to that at meerfelder maar however in this case the final magma resided in the chamber for between 23 days and 2 7 months prior to eruption allowing enough time for any entrained peridotite xenoliths to be filtered out the cumulate xenoliths from the baarley locality record a much longer history than those at the other localities in this case initially peridotitic olivine has been completely re equilibrated over 175 years prior to eruption in common with the other two localities there is evidence of multiple intrusive events at depths up to 7 km prior to final eruption the duration of the final eruption event and the interpreted depth to source of the xenoliths indicate an emplacement velocity of around 3 km h the data derived from modelling of diffusion profiles in olivine from cumulate xenoliths indicate that the magmatic systems feeding these three volcanoes were dynamic and very short lived in addition these data can be used to place constraints on the rate of cumulate formation in subvolcanic magma chambers on the basis of the data presented here it would seem that an appreciable thickness of cumulates can form over relatively short time spans days to a few years

a spectrum of disequilibrium melting preserved in lava hosted partially melted crustal xenoliths from the wudalianchi volcanic field ne china abstract disequilibrium melting has been established as a common process occurring during crustal anatexis and thus demonstrates that crustal assimilation is not a closed system observations of extreme compositional heterogeneity within partial melts derived from crustal xenoliths have been documented in several recent examples however the retention or transfer of elements to and from residues and glasses and their relative contributions to potential crustal contaminants warrants further investigation recent eruptive products of the huoshaoshan volcano in the holocene wudalianchi volcanic field of northeast china contain crustal xenoliths which preserve a spectrum of partial melting both petrographically and geochemically thus providing an excellent natural example of crustal anatexis correlations exist between the volume of silicic glass preserved within the xenoliths and bulk rock sio2 70 83 wt al2o3 8 16 wt glass 87sr 86sr 0 715 0 908 abundances of elements common in feldspars and micas sr ba rb and elements common in accessory minerals y zr nb these correlations are likely associated with the consumption of feldspars and micas and the varying retention of accessory phases during partial melting the xenoliths which contain greater volumes of silicic glass and residual quartz interpreted as being the most melted were found within pahoehoe lava whilst the least melted xenoliths were found within scoria of the summit cone of huoshaoshan thus it is interpreted that the extent of melting is linked to the immersion time in the lava which we estimate to be on the order of days however the least partially melted samples display geochemical evidence in support of melt loss evidenced by the relatively depleted nature of their whole rock compositions and the larger difference between whole rock and glass yet the more melted samples appear to have experienced less melt loss thus the extent of melting cannot be linked linearly to the volume of melt loss which occurs small scale mm mingling and transfer of material from the enclosing lava to the xenolith are observed however modelling of potential contaminant compositions is inconsistent with crustal contamination during lava petrogenesis it is thus more likely that any crustal contamination in these eruptives is extremely localized within this open magmatic system and most likely occurs at the contact zone between the xenolith and host lava

crustal assimilation in basalt and jotunite constraints from layered intrusions abstract to constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers we have modelled published sr and nd isotopic data from three layered intrusions well exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization afc the key to the modelling is that f the mass fraction of magma remaining in the chamber can be estimated from the thicknesses of the studied cumulate sequences this allows afc model curves to be fitted to the isotopic data by varying r the ratio of the rate of mass assimilated to the rate of mass crystallized the results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up section decreases in anorthite content of plagioclase increases in whole rock sr 0 initial 87 sr 86 sr and decreases in whole rock e nd 0 initial e nd the r values of the layered sequences range from 0 12 in the fongen hyllingen intrusion over 0 20 in the bjerkreim sokndal intrusion to 0 27 in the hasvik intrusion the total amount of assimilation the bulk crust magma ratio reaches values of 0 08 0 19 and 0 28 at the level of the most contaminated samples after 60 to 80 crystallisation whereas the instantaneous crust magma ratio of the most contaminated magmas were respectively 0 14 0 46 and 0 70 for the three intrusions innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process the xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions in the hasvik intrusion r 0 27 the initial temperature of the country rocks was 450 c and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated in the bjerkreim sokndal intrusion r 0 20 the country rocks were initially at temperatures of 640 880 c but included both refractory massif type anorthosite and fusible gneisses in the fongen hyllingen intrusion r 0 12 the country rocks were cooler 300 c and the xenoliths include refractory metabasalt dominant and fusible metapelite we argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks resulting in reduced r values in fongen hyllingen and bjerkreim sokndal intrusions heating of refractory and fusible xenoliths and melting of fusible xenoliths absorbed sensible and latent heat of the magma energy balanced modelling shows that up to 75 of the heat available was absorbed by xenoliths within the magma chambers promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone the high r values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves and their constancy reflects the ready availability of fusible xenoliths

caught in the act the first few hours of xenolith assimilation preserved in lavas of the rockeskyllerkopf volcano west eifel germany article i nfo article history lithic sedimentary xenoliths in magmatic pyroclastic deposits in the rockeskyllerkopf volcanic complex of the quaternary west eifel volcanic field preserve evidence of the earliest stages of melting reaction with and assimilation into their silica undersaturated basanite host the xenoliths have a 0 5 1 5 mm thick surface glaze of rhyolitic glass and internally have both silica undersaturated and silica saturated glasses that are the result of in situ melting of micas and clay minerals and reaction of these melts with the surrounding quartz matrix mixing between the rhyolite glaze and host basanite resulted in the formation of a clinopyroxene and glass rich hybrid zone the glazed xenoliths are commonly found with either no coating of lava or with void space between them and the host this unusual occurrence is due to vesiculation of the glaze during eruption which led to physical separation of the xenolith from its surrounding melt simple calculations indicate that assimilation occurred at 1180 c over a period of 12 h and that all the textural features including deformation of the partially molten xenoliths occurred during transport and eruption rather than after deposition

water content and nature of solutes in shallow mantle fluids from fluid inclusions abstract this study discusses new and published data on the composition of fluid inclusions contained in mantle minerals of spinel and garnet peridotite xenoliths in samples from geodynamically distinct settings ethiopian plateau hawaii canary islands and western mediterranean region based on spectroscopic raman and ftir analyses we show that contrary to a commonly held view fluid inclusions either contain relevant amounts of unsuspected h2o or represent a dehydrated composition from multicomponent aqueo carbonic fluids we identify water loss from fluid inclusions through decrepitation stretching and hydrogen diffusion we also show that talc magnesite chlorides and sulfates represent common phases in fluid inclusions talc and magnesite form through reactions of fluids with the surrounding minerals thermodynamic modeling in the mfshc system of observed reactions between fluid inclusions and surrounding mantle minerals provides the basis for predicting water amounts in shallow mantle fluids and suggests x h 2 o in the range of 10 50 mol model hydrous fluids are relatively enriched solutions dominated by si cl and alkalies with significant amounts of ca and s and low levels of mg and fe this study argues that multicomponent hydrous fluids may be widespread in the shallow mantle not only in subduction zones but also in intraplate and extensional settings

exploding xenoliths and the absence of elephants graveyards in granite batholiths abstract biotite monzogranite near the eastern margin of the late devonian south mountain batholith at portuguese cove nova scotia surrounds a large 20 m 25 m anisotropic decimetre scale interbedded psammite and semipelite xenolith of the metamorphic regional greenschist thermal cordierite hornfels country rocks of the cambro ordovician meguma group the xenolith has two distinct types of surfaces against the monzogranite i one rounded diffuse and apparently old surface that had prolonged contact with the granite magma and ii two straight sharp and apparently new surfaces that had more recently come into contact with the granite magma more than 375 thin sheets width n 224 0 1 0 5 cm n 66 0 5 1 0 cm n 51 1 0 5 0 cm n 30 5 0 10 0 cm n 3 10 cm n 2 ranging in composition from monzogranite to pegmatite to coarse quartz sericite cut the xenolith parallel to bedding producing a cumulative volume expansion of 14 in the xenolith aspect ratios in some of the thin sheets exceed 1000 1 several sheets cut the xenolith perpendicular to bedding producing a volume expansion of

crustal xenoliths from calbuco volcano andean southern volcanic zone implications for crustal composition and magma crust interaction crustal xenoliths in the 1961 andesite flow of calbuco volcano in the southern southern volcanic zone ssvz of the andes consist predominantly of pyroxene granulites and hornblende gabbronorites the granulites contain plagioclase pyroxene magnetite amphibole and have pronounced granoblastic textures small amounts of relict amphibole surrounded by pyroxene plagioclase magnetite glass symplectites are found in some specimens these and similar textures in the gabbronorites are interpreted as evidence of dehydration melting mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism a moderate pressure high temperature stage accompanied by melting and melt extraction from some samples followed by thermal metamorphism after entrainment in the calbuco andesite lavas high nd t values 4 0 to 8 6 nd isotope model ages of 1 7 2 0 ga and trace element characteristics like chondrite normalized la yb and la nb 1 indicate that the protoliths were oceanic basalts similar oceanic metabasalts of greenschist to amphibolite facies are found in the paleozoic metamorphic belt that underlies the chilean coastal ranges mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate crystallizing from the basaltic andesite magma at calbuco crystallization pressures for the gabbros based on total al contents in amphibole are 6 8 kbar these pressures point to middle to lower crustal storage of the calbuco magma neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of calbuco andesites although an ancient sedimentary contaminant is indicated by the lava compositions the presence of oceanic metabasaltic xenoliths together with the sedimentary isotopic imprint suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt which is an accretionary complex of intercalated basalts and sediments that formed along the paleozoic gondwanan margin if this is the case the geochemical composition of the lower and middle crust beneath the ssvz is significantly different from that of most recent ssvz volcanic rocks

geochemical evidence for a subducted infant arc in franciscan high grade metamorphic tectonic blocks some of the evidence of tectonic and chemical processes that occur in subduction zones is preserved in blueschists and eclogites as validated by the geochemical data presented from high grade metamorphic tectonic blocks from the franciscan complex in central california combining major elements compatible and incompatible trace element concentrations and ratios and sr nd pb isotopic data we conclude that the protoliths of these high grade franciscan blocks were arc lavas with no continental crust derived components although the protoliths underwent high grade metamorphism at the inception of subduction the tectonic affi nity of the protoliths can be distinguished by our geochemical study our geochemical data are signifi cant in that 1 the protoliths of these high grade blocks have a different mode of origin than the protoliths of the lower grade franciscan basalts that were incorporated later in the subduction complex as the latter generally are considered to be oceanic island basalts oib and derived from mid oceanicridge basalt morb and 2 the protoliths of the high grade blocks were formed over a pre franciscan subduction zone based on the geochemical evidence for an arc origin and the available geochronological data we suggest that protoliths of the highgrade tectonic blocks of the franciscan complex and the coast range ophiolite formed in the same infant arc setting initiation of franciscan subduction in this arc crust led to the formation of the high grade blocks beneath the young coast range ophiolite continued subduction resulted in subduction of older oceanic crust of morb and oib origin some of which was scraped off to form lower grade metabasites within the franciscan complex

precise timing of abrupt increase in dust activity in the middle east coincident with 4 2 ka social change the extent to which climate change causes significant societal disruption remains controversial an important example is the decline of the akkadian empire in northern mesopotamia 4 2 ka for which the existence of a coincident climate event is still uncertain here we present an iranian stalagmite record spanning 5 2 ka to 3 7 ka dated with 25 u th ages that provide an average age uncertainty of 31 y 1 we find two periods of increased mg ca beginning abruptly at 4 51 and 4 26 ka and lasting 110 and 290 y respectively each of these periods coincides with slower vertical stalagmite growth and a gradual increase in stable oxygen isotope ratios the periods of high mg ca are explained by periods of increased dust flux sourced from the mesopotamia region and the abrupt onset of this dustiness indicates threshold behavior in response to aridity this interpretation is consistent with existing marine and terrestrial records from the broad region which also suggest that the later longer event beginning at 4 26 ka is of greater regional extent and or amplitude the chronological precision and high resolution of our record indicates that there is no significant difference at decadal level between the start date of the second larger dust event and the timing of north mesopotamia settlement abandonment and furthermore reveals striking similarity between the total duration of the second dust event and settlement abandonment the iranian record demonstrates this region s threshold behavior in dust production and its ability to maintain this climate state for multiple centuries naturally

local equilibrium of mafic enclaves and granitoids of the turtle pluton southeast california mineral chemical and isotopic evidence free to read on publisher s website major element and trace element compositions of whole rocks mineral compositions and rb sr isotopic compositions of enclaveand host granitoid pairs from the early cretaceous calc alkalineturtle pluton of southeasterncalifornia suggest that the local environment profoundly affects some enclave types in the turtle pluton where the source of fine grained mafic enclaves can be deduced to be magmatic by the presence of partially disaggregated basaltic dikes mineral chemistry suggests partial or complete local equilibrium among mineral species in the enclaveand its host granitoid because of local rb sr isotopic equilibration betweenfine grained enclaves and host granitoid one cannot use sr isotopes to distinguish an enclave source independent of its host rocks from an enclave sourcerelated to the enclosing pluton however preliminary nd isotopic data suggestan independent mantle source for enclaves

reconstruction of glacier variability from lake sediments reveals dynamic holocene climate in svalbard abstract the arctic is warming faster than anywhere else on earth holocene proxy time series are increasingly used to put this amplified response in perspective by understanding arctic climate processes beyond the instrumental period however available datasets are scarce unevenly distributed and often of coarse resolution glaciers are sensitive recorders of climate shifts and variations in rock flour production transfer this signal to the lacustrine sediment archives of downstream lakes here we present the first full holocene record of continuous glacier variability on svalbard from glacier fed lake hajeren this reconstruction is based on an undisturbed lake sediment core that covers the entire holocene and resolves variability on centennial scales owing to 26 dating points a toolbox of physical geochemical xrf and magnetic proxies in combination with multivariate statistics has allowed us to fingerprint glacier activity in addition to other processes affecting the sediment record evidence from variations in sediment density validated by changes in ti concentrations reveal glaciers remained present in the catchment following deglaciation prior to 11 300 cal bp culminating in a holocene maximum between 9 6 and 9 5 ka cal bp correspondence with freshwater pulses from hudson strait suggests that early holocene glacier advances were driven by the melting laurentide ice sheet lis we find that glaciers disappeared from the catchment between 7 4 and 6 7 ka cal bp following a late hypsithermal glacier reformation around 4250 cal bp marks the onset of the neoglacial supporting previous findings between 3380 and 3230 cal bp we find evidence for a previously unreported centennial scale glacier advance both events are concurrent with well documented episodes of north atlantic cooling we argue that this brief forcing created suitable conditions for glaciers to reform in the catchment against a background of gradual orbital cooling these findings highlight the climate sensitivity of the small glaciers studied which rapidly responded to climate shifts the start of prolonged neoglacial glacier activity commenced during the little ice age lia around 700 cal bp in agreement with reported advances from other glaciers on svalbard in conclusion this study proposes a three stage holocene climate history of svalbard successively driven by lis meltwater pulses episodic atlantic cooling and declining summer insolation

radioisotopic and biostratigraphic age relations in the coast range ophiolite northern california implications for the tectonic evolution of the western cordillera the coast range ophiolite cro in northern california includes two distinct remnants the elder creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites layered gabbro wehrlite pyroxenite quartz diorite a mafi c to felsic dike complex and mafi c felsic volcanic rocks the entire suite is cut by late mid oceanic ridge basalt morb dikes and overlain by ophiolitic breccia the stonyford volcanic complex sfvc comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite structurally below this seamount are melange blocks of cro similar to elder creek u pb zircon ages from plagiogranite and quartz diorites at elder creek range in age from 165 ma to 172 ma u pb zircon ages obtained from cro melange blocks below the sfvc are similar 166 172 ma 40 ar 39 ar ages of alkali basalt glass in the upper sfvc are all younger at 164 ma radiolarians extracted from chert lenses intercalated with basalt in the sfvc indicate that the sedimentary strata range in age from bathonian unitary association zone 6 6 of baumgartner et al 1995a near the base of the complex to late callovian to early kimmeridgian unitary association zones 8 10 in the upper part the sfvc sedimentary record preserves evidence of a major faunal change wherein relatively small sized polytaxic radiolarian faunas were replaced by very robust oligotaxic nassellarian dominated faunas that included praeparvicingula spp we suggest that cro formation began after the early middle jurassic 172 180 ma collision of an exotic or fringing arc with north america and initiation of a new or reconfi gured east dipping subduction zone the data show that the cro formed prior to the late jurassic nevadan orogeny probably by rapid forearc extension above a nascent subduction zone we infer that cro spreading ended with the collision of an oceanic spreading center ca 164 ma coincident with the oldest high grade blocks in the structurally underlying franciscan assemblage we further suggest that the classic nevadan orogeny represents a response to spreading center collision with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in north american plate motion to rapid northward drift j2 cusp causing sinistral transpression and transtension in the sierra foothills these data are not consistent with models for late jurassic arc collision in the sierra foothills or a back arc origin for the cro