deep basement seismic reflection profiling of the purcell anticlinorium using a land air gun source seismic reflection profiling in the purcell anticlinorium of southern british columbia canada using a land air gun source has revealed reflections from depths of at least 15 20 km interpretations of these data in conjunction with related geological and geophysical information confirm the allochthonous structure of the purcell anticlinorium and suggest that autochthonous north american basement at 15 20 km depth beneath the anticlinorium has a westward dip of 15 20 the thickness of the crust above the north american basement surface may be accommodated by structural repetition of purcell strata by thrust slices of north american basement rocks or by both the geometry observed on the reflection data implies that the purcell anticlinorium is cored by a crustal scale thrust anticline hanging wall anticline further the data demonstrate that land air guns are effective sources for crustal reflection work

anomalous paleomagnetism of the crowsnest formation of the rocky mountains the volcanic crowsnest formation of albian age late early cretaceous from the rocky mountain fold and thrust belt of alberta has a stable remanent magnetization with a mean direction of 349 59 95 5 and paleopole at 78 n 108 e dm 7 dp 5 the inclination is lower than and the declination clockwise of the expected mid cretaceous paleogeomagnetic field for cratonic north america taken at face value the result indicates that the crowsnest formation and the thrust sheet in which it occurs have been transported from the south relative to cratonic north america by 17 6 about 1800 km and rotated 24 10 clockwise it is also possible that flattening of inclination is caused by magnetic anistropy but tests show this to be unlikely a third possibility is that the magnetization is secondary and of latest cretaceous age but there are good reasons for believing this is not so lastly it is possible that the unit could have been formed close to its present position relative to the craton

geochemical mineralogical and isotopic data relating to the origin and tectonic setting of the rossland volcanic rocks southern british columbia bulk rock and mineral chemical and isotopic analyses of rossland volcanic rocks are used to infer the nature of the magma extruded in the nelson rossland area of southern british columbia during th

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

the cenozoic denali fault system and the cretaceous accretionary development of southern alaska the juxtaposition of disparate geologic terranes in southern alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the denali fault system in cenozoic time recent geologic investigations in the healy quadrangle strongly suggest that cenozoic horizontal displacements of such magnitude along the denali fault system do not exist in the healy quadrangle isograds and metamorphic facies boundaries of an early late cretaceous metamorphic belt trend across the cenozoic mckinley strand of the denali system without significant horizontal offsets the present geologic makeup of most of southern alaska is primarily the result of the talkeetna superterrane consisting of the previously assembled peninsular terrane and wrangellia colliding with and subsequently being thrust upon the yukon tanana and nixon fork terranes of the ancient north american continent in about middle cretaceous time the leading edge of the talkeetna superterrane faces a wide complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with jurassic and cretaceous flysch the flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks the postcollisional denali fault system developed in cenozoic time across the already accreted continental margin in eastern alaska along an older cretaceous suture

eocene underplating along the kodiak shelf alaska implications and regional correlations structural geology and geophysical data from the kodiak shelf suggest that the mesozoic rocks exposed on the shelf are structurally underlain at about 12 km depth by several kms of eocene age strata kinematic data from the late cretaceous to paleocene ghost rocks formation indicate that this formation and probably all of the kodiak islands were uplifted vertically to nearly their present elevations landward tilting and imbrication are not indicated the age of uplift is indicated by a regional angular unconformity of early eocene to early oligocene age that separates deep sea rocks from shallow water to non marine rocks the uplift of the accretionary prism is believed to have been caused by underplating of an eocene sedimentary sequence because 1 a band of seismic reflections that occur 12 to 20 km beneath the shelf is interpreted as the top of the underplated material and 2 an obductively offscraped sequence of eocene deep sea rocks crops out on the seaward side of the kodiak shelf suggesting that a thick trench fill sequence may have been present prior to uplift of the prism the underplated material is interpreted to be part of either a previously unrecognized turbidite fan of early eocene age or a proximal equivalent of the zodiac fan of late eocene to early oligocene age other possible on land remnants of the underplated material may be present in prince william sound the montague belt the gulf of alaska lower sections of the yakutat block and in the coast ranges of oregon and washington the large volume of underplated material beneath the kodiak shelf suggests that underplating may be the dominant process in the growth of convergent margins

cordilleria revisited with a three dimensional model for cretaceous tectonics in british columbia the cordilleria microcontinent theory for the tectonics of british columbia in the cretaceous and early tertiary and for the origin of the canadian rocky mountains and related features is reviewed new isotopic geological and paleomagnetic data support the hypothesis that the amalgamated superterranes of british columbia and alaska were not attached to the mainland of north america in the early cretaceous and were probably well south of their present relative position consideration of that evidence together with sedimentological evidence from the albian leads to a slightly earlier best estimate 115 ma for the time of initial collision in the yukon new geophysical data for southeastern british columbia and idaho lead to a comparison with the iapetus suture in northern england it is suggested that the cordilleria suture dips west from the present purcell thrust and related structures and should cut the moho a new model for regional deformation in se british columbia is proposed involving a flake

structural morphology and associated strain in the central cordillera british columbia and washington evidence of oblique tectonics the central cordillera developed during early jurassic to late cretaceous time as the continental edge was subducted under the exotic terranes of quesnellia and stikinia the earliest jurassic structures formed during prograde metamorphism and include cylindrical and double verging folds that trend subparallel to the orogen these structures are associated with mineral lineations and stretch values between 120 and 400 parallel to the fold axes later disharmonic structures formed during the emergence of the crust after the metamorphic peak and in response to decreasing p t conditions present tectonic models do not incorporate the kinematics capable of forming the early structures and thereby only address the emergent phase a scenario is offered which involves oblique subduction of the craton edge and which is kinematically compatible with the early structural morphology and associated strains

the use of drill bit energy as a downhole seismic source a new wellbore seismic technique uses the vibrations produced by a drill bit while drilling as a downhole seismic energy source the technique is described as inverse vsp because the source and receiver positions of conventional vsp are reversed no downhole instrumentation is required to obtain the data and the data recording does not interfere with the drilling process these characteristics offer a method by which borehole seismic data can be acquired processed and interpreted while drilling interchanging the conventional vsp source and receiver positions improves the efficiency of recording multioffset surveys for imaging a 3 d data volume in the borehole vicinity the continuous signals generated by the drill bit are recorded by a pilot sensor attached to the top of the drillstring and by receivers located at selected positions around the borehole the pilot signal is crosscorrelated with the receiver signals to compute traveltimes of the arrivals and to attenuate incoherent noise deconvolution and time shifts of the pilot signal compensate for the effects of propagation from the drill bit to the top of the drillstring by repeating this process for an interval of the well a vsp equivalent data set is generated results from a test well demonstrate that the processed drill bit data are comparable to conventional vsp data

mise en evidence d un cisaillement ductile dextre d âge cretace moyen dans la region de tete jaune cache nord est du complexe metamorphique shuswap colombie britannique the boundary between the external zones rocky mountains and the internal zones of the eastern canadian cordillera is marked by a tertiary half graben the rocky mountains trench rmt in the south cordillera east of the shuswap metamorphic complex the fault limiting the trench is superimposed on an early major thrust the late jurassic purceli thrust on approaching this discontinuity the ductile deformation of the miette group a detrital precambrian suite is characterized by a subvertical foliation and a subhorizontal stretching lineation parallel to the fold axes the deformation intensity its noncoaxial characters and its geographic extension are interpreted as resulting from a dextral crustal shear parallel to the mapped trace of the purcell thrust and rmt the dextral slip is deduced from a microtectonic analysis of the observed rotational criteria and is consistent with the small angle occurring between the directions of the linear structure stretching lineations and fold axes and those

nd evidence for proterozoic crustal development in the belt purcell supergroup a knowledge of the chemical and isotopic characteristics of the continents as they existed in the past is essential to an understanding of their evolution1 7 vestiges of these characteristics remain in the sedimentary record for example the character and crustal residence age4 of ancient continental crust has been deduced from the sm nd isotopic patterns of clastic sediments that have survived to the present day4 6 8 we report here sm nd isotopic measurements from the belt purcell supergroup of western north america which form the first detailed isotopic investigation of a major proterozoic sedimentary succession the belt purcell has many features also seen in the detrital load of the world s major contemporary rivers including exceptionally homogeneous 147sm 144nd and 143nd 144nd ratios and a small range of crustal residence ages mantle additions to the continental crust seem to have been relatively minor since the middle proterozoic

geochemistry and geochronology of the gneisses east of the southern rocky mountain trench near valemount british columbia petrographic geochemical and geochronological data are presented on the gneisses of the bulldog creek block the mount blackman block and the hugh allan creek block which lie to the east of the southern rocky mountain trench srmt south of valemount british columbia petrographical and geochemical data especially immobile trace element ratios nb y ti zr and cao versus y and afm plots are used to deduce the probable origins and protoliths of the gneisses the mount blackman block consists of a psammitic paragneiss probably derived from an immature arkosic sedimentary protolith intruded by sills of tholeiitic basalt now amphibolites the bulldog creek block consists of felsic orthogneisses of calc alkaline affinity which are structurally concordant with mafic orthogneisses of possible tholeiitic basalt parentage the hugh allan creek block consists of a felsic orthogneiss with a probable alkaline granite protolith rb sr and some u pb analyses show that each block has experienced a separate geological history the mount blackman block psammitic paragneisses are the only analysed gneisses east of srmt with a probable archean rb sr model crustal residence age u pb analyses on zircons from these gneisses give a 1950 ma minimum source rock age and rb sr whole rock analyses suggest a 1860 50 ma age for amphibolite facies metamorphism of both paragneisses and amphibolites the bulldog creek block gneisses have a metamorphic age of at least 640 ma but their rb sr systematics have been extensively disturbed possibly during mesozoic retrogressive metamorphism the hugh allan creek block gneisses have a rb sr model crustal residence age of 900 ma and a metamorphic age of 805 11 ma it is not possible to correlate any of these lithologies or events across the srmt with the malton block and it is concluded that the srmt is the site of a major fault or faults at this latitude nous prksentons les donnkes pktrographiques gkochimiques et gkochronologiques sur les gneiss des blocs bulldog creek mount blackman et hugh allan creek lesquels sont situks i i est du sillon des rocheuses du sud au sud de valemount en colombie britannique

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

petrology and tectonic significance of gates formation early cretaceous sediments in northeast british columbia moosebar gates sandstones are predominantly litharenites with some feldspathic litharenites both the light and heavy mineral suites indicate a mixed source characterized by clastic and carbonate sedimentary rocks acidic to intermediate plutonic and volcanic igneous rocks and metamorphic rocks the sediment sources all fall within a recycled orogenic provenance grouping histograms showing stratigraphic variation of mineral content do not indicate any significant progressive unroofing of more deeply buried source rocks the source area was very extensive regionally and extended well into the omineca crystalline belt and eastern margins of the intermontane belt zebraic chalcedony was derived from evaporitic rocks of the charlie lake formation situated east of the rocky mountain trench kyanite and almandine garnet were probably derived from the omineca crystalline belt west of the rocky mountain trench regional paleoslope dipped towards the north northwest restoration of strike slip on the rocky mou

metamorphism of the spokane dome mylonitic zone priest river complex constraints on the tectonic evolution of northeastern washington and northern idaho metamorphic rocks in the core of the southern priest river complex record two distinct metamorphic events the earliest metamorphism occurred above 5 kb near the amphibolite granulite boundary and accompanied the formation of a steep crystalloblastic foliation the second less pronounced metamorphism was at lower pressure 3 5 kb in the middle amphibolite facies and accompanied mylonitization within a 4 km thick zone retrograde reactions during the second event include the formation of andalusite muscovite secondary green biotite and the breakdown of garnet biotite garnet geothermometry yielded widely variable and unrealistically high temperatures plagioclase garnet geobarometry is partially reset by the retrograde event and therefore provides minimum pressures the p t path is not known precisely but may include two episodes of prograde metamorphism perhaps related to continued accretion of terranes to the west the first metamorphism probably accompanied the jurassic accretion of quesnell

paleomagnetic evidence for displacement from the south of the coast plutonic complex british columbia the mid cretaceous spuzzum and porteau plutons of the coast plutonic complex of british columbia have two magnetizations a and b the a magnetization eight sites 83 specimens d 30 3 i 56 7 95 4 9 paleolatitude 37 5 n paleopole 65 0 n 14 9 w a95 6 2 is considered to have been acquired in the age range 105 90 ma this result differs from the field established for cratonic north america in this time range the difference could be caused either by previously undetected tilting about a horizontal axis of the plutons or by their rotation about a vertical axis and lateral displacement relative to the craton previously observed mid cretaceous magnetizations from other rock units from the western canadian cordillera and the cascades of washington united states are similarly discordant with respect to the craton this similarity over such a large area indicates that although local undetected tilting could be partly responsible it is unlikely to be the prime cause and we argue ther

relationships between plate motions and late cretaceous to paleogene magmatism in southwestern alaska four major late cretaceous to paleogene magmatic belts have been recognized in southwestern alaska and tentative boundaries have been drawn for each the belts indicate minimum ages by which the tectonostratigraphic terranes they overprint must have been linked together though there has been significant postmagmatic tectonism the kuskokwim mountains belt 73 60 m y b p the alaska range belt 74 55 m y b p and the gulf of alaska belt 62 57 m y b p all may have been related to a period of rapid north northeastward motion of the kuia plate from about 74 to 56 m y b p the angle of convergence was low with respect to the present orientation of the belts suggesting the possibility of postmagmatic rotation of the belts a higher angle of convergence may have existed with respect to the alaska range belt a probable arc if postmagmatic rotation resulted from 300 400 km of right lateral displacement on the denali fault the gulf of alaska belt is in an anomalous forearc position with respect to the alaska range belt but probably formed by a convergence related mechanism the kuskokwim mountains belt is in an apparent back arc position but the mechanism of its origin is uncertain it may have been part of an unusually broad alaska range arc or less likely it may have been a separate arc magmatic quiescence occurred throughout south western alaska during a period of plate reorganization from about 56 to 43 m y b p a period which began with extremely rapid northward motion of the kuia plate magmatism resumed when the present period of rather slow northwestward pacific plate motion began at about 43 m y b p the aleutian arc probably formed at this time and has been episodically active in about the same location ever since

cordilleria a newly defined canadian microcontinent palaeomagnetic data and geological field evidence indicate that most of the western canadian cordillera lay 1 500 km south of its present position in the early cretaceous we suggest that this region comprised a microcontinent termed cordilleria which moved on the kula plate and collided with the craton 100 myr ago to form first the mackenzie mountains and then the rockies

offset of an upper pliensbachian geographic zonation in the north american cordillera by transcurrent movement the north to south climatic zonation of upper pliensbachian ammonites known in europe has been recognized in the north american cordillera this simple geographic arrangement has been disrupted by northwest trending dextral transcurrent faults with further documentation the amount of offset of these zones should provide a measure of the amount of movement on these faults since pliensbachian time and further document the allochthoneity of the western part of the cordillera

imbricated terranes of the cariboo gold belt with correlations and implications for tectonics in southeastern british columbia the cariboo gold belt of east central british columbia is divided into four fault bounded sequences of distinct stratigraphy they are from east to west the cariboo continental shelf sediments barkerville continental shelf sediments and intercalated volcanics slide mountain rift related submarine pillow basalt chert and diorite and quesnel island arc sediments and subaqueous volcanics terranes each is separated from others by thrust faults grit phyllite limestone and volcanics of the barkerville terrane may be correlative with the eagle bay formation near adams lake and the lardeau group near kootenay lake barkerville terrane may be part of a more regional rock package selkirk terrane which is defined to include kootenay terrane badshot formation and horsethief creek and hamill groups selkirk terrane is i separated everywhere by a low angle fault from the overlying age equivalent but stratigraphically and structurally different cariboo terrane and ii separated by a system of f

nd evidence for extensive archean basement in the western churchill province canada to determine the extent of reworked archean crust in the western churchill province we have examined sm nd crustal residence ages of basement cores in southern alberta and southwestern saskatchewan along with crustal residence ages calculated for other sm nd data available from the churchill province the deep drill hole samples from the interior platform give sm nd crustal residence ages that average 2 8 ga granulites from northeastern alberta and composite gneisses from northern saskatchewan also provide archean crustal residence ages these data demonstrate that the presence of reworked archean crust in the western churchill province is more widespread than has been confirmed previously we suggest that sm nd crustal residence ages are valuable estimates of crust formation times especially in orogenic areas where other isotope systems have been disturbed

late cretaceous genesis of the kula plate abstract we propose that the pacific kula ridge began spreading approximately 85 m y b p during late cretaceous time extrapolation of the great magnetic bight backwards in time results in an implausible ridge configuration this implies that plate velocity vectors for the pacific kula and farallon plates were not constant during this interval evidence for splitting of the kula plate from the pacific plate along the chinook trough is the relationship between the north striking amlia and adak fracture zones the chinook and emperor troughs and the magnetic lineations south of the aleutian trench if this hypothesis is correct it will require that mesozoic reconstructions of the pacific basin and their relation to cenozoic reconstructions be re examined a previously unrecognized mesozoic plate may be required we propose calling this the izanagi plate

holocene coastal evolution under the influence of episodic tectonic uplift examples from new zealand and japan abstract coastal geomorphology in many parts of new zealand and japan has been strongly influenced by tectonic uplift since the culmination of the holocene transgression at 5 5 7 ka ago examples of historic coastal uplift in japan and new zealand accompanied by major earthquakes provide analogues for interpreting coasts where holocene marine terraces are arranged in staircase fashion despite complexities in their origin there is commonly only one shore platform for each relative stillstand of sea level shore platforms are subhorizontal and are usually graded to low tide level the width of platforms depends primarily on the erodibility of bedrock and the length of time available for formation the simplest setting for assessing the influence of uplift on coastal evolution requires readily erodible homogeneous bedrock and a microtidal environment so that wide shore platforms of limited surface relief can be cut within 1 0 m of mean sea level earthquake related uplift where uplift is greater than the tidal range is an instantaneous and catastrophic event that overwhelms on going processes of shore platform formation determining the degree of tectonic control on coastal evolution is more difficult where hard or layered dipping bedrock exists or where the coast is subject to spatial variation in wave energy temporal and spatial variation in wave energy and variation in sediment supply make beach ridges more difficult to relate to former sea level positions than either hard or soft bedrock platforms more precise measurement of past uplift events are possible in coral reef environments than at higher latitude sites because coral reef colonies grow within 0 10 m of the mean low water spring tide and are sensitive to sea level changes

the anvil plutonic suite faro yukon territory the anvil plutonic suite consists of three phases a peraluminous muscovite biotite granite mount mye phase and two metaluminous to peraluminous hornblende biotite granodiorite and minor granite intrusions orchay and marjorie phases the suite is massive or foliated equigranular or seriate and contains alkali feldspar megacrysts the marjorie phase is characteristically porphyritic geochemical trends are irregular for the suite and for individual phases high k2o low cao and low mgo compositions typify the silicic calc alkaline suite hornblende bearing phases contain less sio2 k2o and rb and more cafemic oxides tio2 sr ba and y than the mount mye phase and are compositionally similar to coeval south fork volcanics isochrons from some of the orchay phase whole rock samples t 99 2 5 ma 87sr 86sri 0 7161 0 0001 and from whole rocks and minerals of the mount mye phase t 100 2 ma 87sr 86sri 0 7405 0 0001 indicate they are coeval but not comagmatic accounting for the litho