325

The Catndian Mineralogist Vol. 34, pp.325-334 (1996)

CONSTRAINTSON THEMANTLE PLUME MODEL FORMESOZOIC ALKALINE INTRUSIONS IN NORTHEASTERNNORTH AMERICA

J. GREGORY McHONE GraduateLiberal StudiesProgram" WesleyanUniversity, Miildletoywt, Comtecticut 06459-0519, U.S.A.

AssrRAcr

Mesozoicalkaline intrusionsoccur with elongatepattems of distributionin southernQuebec and centralNew England"and similar magmasfonned the chain-likeNew Englandseamounts in the adjacentNorth Atlantic Oceanbasin. Both igneousgroups lsvs b€enlinked by a "" model in which the North Americanplate overrodea stationarymantle plume between125 and 70 million years ago. Evidence for a hotspot plume includes mantle enrichmentfollowed by melting to produce alkaline m2gmas,a progressionofradiometric agesfor the New Englandseamounts, and an apparentorientation ofintrusions witl plate motion. In keepingwith the popularity of a mantleplume origin for all seamountchains, the model of a New Englandhotspot plume is often describedas a demonstratedfact rather than as one of severalhypotheses, Closer study reveals that major goblems exist with a hotspot link betweenthe seamountsand continentalalkaline rccks in nortleastern North America. In particular, many alkaline igneousintrusions in northeasternNorth America do not fit any plume track in age or geographic distribution,despite petrological similsritigs that imply a commonorigin. A deepcolumnar plume cannotbe the principal cause of magmatism;rather, the geological evidencecalls for widespread,heterogeneous source-areas in the mantle that produce alkalinebasalts in concertwith the tectonicre-activation of lithosphericstructues, atlowing isolatedalkaline intrusions as well as linear chainsto fonn throughspace and time,

Keywords:hotspog New Englandseamounts, , Cretaceousalkaline plutons, ,New England.

SoMMeRs

Les complexesalcalins d'Sgsmesozoique ddfinissent une distribution quasi-lindairedans le sud du Qu6becet la partie centralede la Nouvelle-Angleterre,et dessuites magmatiques semblables ont 6t6 misesen place dansune s6riede guyotsdans le bassinadjacent de I'oc6anAtlantique. Cesdeux groupesde rochesign6es seraient des manifestations d'un seul point chaud. Selon ce modble, la plaque tectonique de I'Am6rique du Nord se serait d6plac6elat6ralement par dessusun panache asth€nosph6riquestationnaire etrtre 125 et 70 million d'ann6espass6es. Sont citds en 6videnced'un tel panacheun enrichis- sementg6ochimique du manteau,suivi d'unefusion partielle, pour expliquerla formation de magmasalcalins, une progression des dgesradiom6triques dans la chalne de guyots de la Nouvelle-Angleterre,et un alignementdes complexesintrusifs en concordanceavec la direction du d6placementde la pliaque.Comme lhypothdse d'un panacheasth6nosphdrique est celle qui est Laplus courammentretenue pour expliquer toutes chalnesde guyots, on traite souventce moddle de fait 6tabli, plut6t que dhypothdseparmi d'autes. Une 6tudeplus approfondierdvdle une s6rie de probldmesimportants avec I'id6e que les guyots et les complexesmagmatiques alcalins du nord--€stde I'Am6rique du Nord r6sultent d'un point chaud dans le manteau. En particulier, plusieurs des complexesintrusifs alcalins de ce secteurtre concordepas avec le trac6 propos6d'un panache asth6nosphdrique,soit par leur dge ou leur distribution g6ographique,malg€ leurs ressemblancesp€trologiques qui supposent une origine commune.Un tel panachene pourrait pas 6tre la causeprincipale du magmatisme;les observationsg6ologiques requierent plut6t des r6gionsdu manteauhdtirogbnes et rdgionales,propices i la production de magoas basaltiquesalcalins lors de la r6activationtectonique de structureslithosph6riques. Ces circonstaacespermettraient la formation dansI'espace et dansle tempsde massifsintrusifs alcalins,soit en isolation, soit en chalneslindaires. (Traduit par la R6daction)

Mots-cl6s:ptnt chaud guyotsde la Nouvelle-Angleterre,collines mont6r6giennes, plutons alcalins cr6tac6s, Qudbec, Nouvelle- Angleterre.

INTRoDUcnoN Although early models related ttrem to ocean transforms and fracture zones (Marsh 1973, Sykes Oceanic volcanoes and occur both as 1978),chains of seamountsare now generallyascribed members of linear chains and as non-oriented clusters to rising mantleplumes beneath moving plates@urke within ocean plates (e.g., Epp & Smoot 1989). & Wilson 1976). Ta.contrast with oceanic regions, 326 TTIE CANADLA,NMINERALOGIST continental volcanic chains that correlate with plate actually in force should work anywhere,in continents motions are uncommon or difficult to recognize, as well as in oceansoand it should suffice for any despite the abundanceof continental alkaline occurrenceof alkaline intraplatemagmas. and plutons.Even more rare are chainsofplutons that could mark hotspot tracks across continent-ocean MEsozorcIcNeous PnovNcss boundaries.One regionin which continentalalkalins plutons are commonly linked with a chain of Mesozoic igneousrocks of easternNorth America seamountsis in New England and adjacent Quebec have been described elsewhere O4cHone & Butler (Coney 1971, Crough 1981, Morgan 1983, Sleep 1984,de Boer et al. L988,Jansa & Pe-Piper1988). 1990).However, there has been a tendencyto over- Although intrusions such as kimberlites and some simplify the evidencefor a hotspot track acrossthe seamountsappear to be relatively isolated, most New England region by ignoring important features plutons are members of geographic and temporal that should apply to the model. This groupsthat occur within boundedprovinces QvlcHone paperis a discussionof problemswith the proposed & Butler 1984). Figure 1 includes Jurassic and link of Cretaceousigneous rocks of the New England- Cretaceousplutonic intrusions and provinces, but it Quebec province and the New England chain of omits the more wide-ranging Early JurassicEastern seamounts. North American tholeiitic dikes and flood basaltsthat By original definition, a oohotspot"is a place where occuralong the entireseaboard (de Boer et al.1988). igneous features (especially volcanoes)have formed Such tholeiitic basaltswere producedduring Jurassic through some mechanismnot related to plate-margin ocean-openingevents that precededmost of the magmatism.The term 'hotspot" is now also used in alkalinehotspot magmns, and they possiblyinclude a referenceto the assumedlocation of a plume within the thick basaltic wedge that is responsiblefor the East mantleas well asto the geographicsurface-point above Coastmagnetic anomaly @CMA on Fig. 1; Holbrook a plume, even where that surface shows no volcanic & Keleman 1993).Although opinionsvary concerning features(e.g., Epp & Smoot1989). The successof tle their origin (e.g., de Boer et al. 1988), the Early plume model in explaining the origin of chains of Jurassictholeiites do not havecharacteristics ofplume- seamountshas led to the proposalthat mantle plumes relatedmagmas. are necessarymechanisms for tle production of Intrusive complexesof the White Mountain magma alkalinebasalts, both continentaland oceanic(Burke & series ftYMMS), mainly of Early Jurassic age, are Wilson 1976). By their nature, mantle plumes are locatedin centraland northemNew Hampshire,where evidencedby a series of volcanoesor plutons as an they form a roughly NNW-trending belt of large age-progressivelinear chain in the direction of plate syenite - monzonite - alkali ganite plutons Gig. 1; motion. However. alkaline volcanic and intrusive McHone & Butler 1984).Most radiometricdates are rocks, particularly in continental areas but also in between190 and 160 Ma, but severalsmaller plutons oceans,commonly occur in isolated suucturesor in the province are consideredto be older by l0 to individual plutons that are not links of an age- 30 Ma (Foland&Furl 1977).Crustal structures (faults progressivechain; these fit only the original (non- and folds) are not correlatedwith the pluton trend of genetic)and not the more recent(mantle plume) usage the JurassicWMMS rocks McHone & Shake 1992). of the term'ohotspot".Also, many alkalinemagmas are Many hundredsof tholeiitic to mildly alkaline basaltic associatedwith continentalrifts that are not causedby dikes, most of which appear to be Early Jurassic, plumes (Moreau et al. 1987, Bradshaw et al. 1993, maintain a northeasterlytrend acrossand among the Anderson1994, Pederson & van der Beek 1994),and WMMS plutons (McHone 1984).The Conwaygranite so some authorsconfine plume-generatedmagmas to is a late memberof both the Early JurassicWMMS and demonstrable volcanic-plutonic age-progressive the more strongly alkaline Early Cretaceousplutons in chains,or "hotspottracls" (Morgan1983). A distinc- New Hampshire, indicating a common origin from tion is then made amongo'types" of hotspots(Sleep partial melting of the samecrust. 1984, Marzocchi & Mulargia 1993), which implies Early Cretaceousmagmatism in southern Quebec different mechanismsfor the origins of very similar and northern New England produced the classic alkalinebasalts. MonteregianHills intrusions@by 1984)in Canada,as Critical argumentsconcerning a hotspot track in well as several complexes in Vermont, New northeasternNorth America should center upon the Hampshire, and southern Maine (McHone & Butler agesand geographicpatterns of its Mesozoisalkaline 1984). Both Si-saturatedand Si-undersaturatedrock igneous rocks. A model of an independentmantle qryes are well represented,with gabbro and syenite plume, as proposedfor only some of the alkaline most cornmon. Several thousand Early Cretaceous magmas in New England, must be reconciled with lamprophyre dikes in the region form a continuous petrological similarities among many other intrusions dike-swarmamong and cross-cuttingall of the coeval widely distributedin spaceand time, but which are not plutons, overlapping the older WMMS province as part of a hotspot track. Whatever mechanism was well @g. 1). McHone& Butler (1984)grouped all of CONSTRAINTS ON MANTLE PLUME MODEL 327

650 600

NEQProvincebundary for Early K igneous rocks s@taie Rldge dtk@&flM

-J- -/& suthem @ild Baks tgnew r6ks / cdnfof I4alre tr ,/ W'?o"!nirdi\""** u't Reuievel i,F Vl G Beart, 'ro!ilr* + Sotm Abyxal nain a &eatstee Dme*'"; Wre. '. c*ess I r .."'' : 3"" .'asilPauo I :,0 l*n****, Ar.r-d"/"{ *nilfooo* , Jl e@nqd O Aileghwy A,B vosS \ l:-narnaet (f t; \-::;y:, .'*;Y -f I AtlanticOcean ""%"\a."*rr"

Frc. 1. Mesozoiccontinental and oceanicigneous features ofnortheastern North America. Plutons,volcanoes and seamounts axeshown in solid black, with heavy lines showingprovince boundaries.Kimberlites and ultramafic intrusionsare located by x symbols.Plutons A through N are inferred from geophysicalanomalies (Klitgord et al. 1988).Late Triassic - Early Jurassicsedimentary basins are shownby shadedzones, and solid lines mark Mesozoicfaults. NEQ is the New England- Quebecigoeous province of McHone & Butler (1984),WMMS is the White Mountainmagma series, and ECMA is the East Coastmagnetic anomaly. Early Jurassictholeiitic basaltsand dikes are not shown.Most featuresare adaptedfrom Figure I ofJansa& Pe-Piper(1988), Plate2C ofKlitgord arai. (1988),and Figure 1 ofSwift eraL (1986).

the Early Cretaceousdikes andplutons under the name basin(McHone & Butler 1984). New England- QuebecQ.{EQ) igneous province. Late Jurassicto Early Cretaceousplutonic provinces Dike and pluton trends vary across the NEQ Grg. 1) also are found in Newfoundland (Strong & province. Major trends are near N65'W in southem Hanis 1974)and westernVirginia @aderel al. 1986). Quebec, almost E-W in west-centralVermont, and The rock typesare remarkably similar to thoseof dikes between N45"E and N60'E throughout southern and plutons in the NEQ province. Kimberlites and Vermont andthe rest of New England(McHone 1984). relatedultramafic dikes are scatteredin a linear belt in The northeasterlytrends of the Cretaceousdikes mimic the Allegheny plateauand westemAppalachians from trendsof earlierJurassic and Triassic dikes in the same Tennesseenortheastward to the western edge of the region. Dike trendsof the NEQ province correlatealso MonteregianHills of Quebec(Fig. 1). The only well- witl some major topogaphic and geophysical establisheddates are for Late Jurassickimberlites in lineaments,and there are strong correlations of the centralNew York State(de Boer et al. 1988). topographic lineaments with regional fracture-zones The New England seamountscomprise an iregular (McHone & Shake 1992). West-northwest-trending chain of many small and large volcanic suboceanic lineamentsthrough the area are parallel to the New mountainsthat extendsat least 1000km southeastward England chain of seamountsof the adjacentAtlantic from the continental rise @g. l). In common with 328 TIIE CANADIAN MINERALOGIST many other mid-plate volcanoes,dredged and drilled dentally within the frack of a later Cretaceousplume. samplesof the New Englandseamounts display mainly Alternatively, a long-lived lithospheric tectonic zone alkaline hypabyssal basalts and volcanic rocks might control the geographic overlap of similar @uncan 1984, Taras & Hart 1987).Initial models ma$nas of different ages, as discussedin a later relatedthe New Englandseamounts to fractureseither section. The generationof basanitic and nephelinitic propagating from the continent or continuous with magmasrequires the generationof enrichedmelts from ocean-ridgetansforms (Jchupi et al. 1970,LePichon mantlewithin or below the baseof the lithosphere,but & Fox 1971,Sykes 1978).The interpretationof the whether or not the same mantle must maintain a New Englandseamounts in termsof a traceof a mantle connection or move with the contingntal plate is plume was proposedby Coney(1971) and developed unclear. by Morgan (197L),Crough (1981), Sleep (1990) and others.This modelreceived a strongendorsement from Thereis little evidenceofage progression Duncan (1984), who found evidence for an age along 400 krn of the NEQ province progression along the chain that appears to be consistentwith plate motions. Most of these authors Paleomagneticpoles measuredfor the Monteregian alse link the track of seamountswith the belts of Hills and selectedplutons through southeastemNew Monteregian and New England Mesozoic plutons. Hampshireare statisticallysimilar (Van Fossen& Kent Aithough commonly describedas a "well-established" 1992),n agrcementwith the short 122-124Ma Ar-Ar hotspot or plume track (de Boer et aL 1988, Sleep age range reported by Foland et al. (t996) ana 1990),other petologists considerthe plume model for Hubacher & Foland (1991) for some of the same these ocean and continentalrocls to be problematic inftusions. The cluster of magneticpoles measuredby (McHone 1981,Lameyre et al. 1984,Swift. et al. L986, Van Fossen& Kent (1992) is also broad enough to Jansa& Pe-Piper1988, Hale & Friberg1995). match other North American paleomagneticpoles as Although attractivein its simplicity and generality young as 88 Ma or about the age of seamounts for explaining innaplate magmas,the hotspot-plume measuredby Duncan (1984) in the central part of the modelfails for someimFortant igneous features of the New Englandchain of seamounts(Fig. 2). New England region. The following observationsare An important observationis that the L22-124 Ma intendedto provide counterpointsto specificaspects of magmatism was apparently concurrent over a large the model for a mantle plume and hotspot track of areaof the continent.OtLer Rb-Sr, K-Ar, and fission- alkaline magnas through New England, across the track measurementsof Monteregian rocks range continent-oceanborder, and into the westernAtlantic between 118 and 136 Ma (Eby 1984), and NEQ Oceanbasin. intusions across central New England have been assigneddates from 101to 136Ma (seethe compila- PnosLEMswrlrl TIIEHorspor Motg- tion by McHone 1984).There is no consistentffend for publishedages of Cretaceousalkaline intrusions in any Many WMMS(Jurassic) mngmas are petrologically directionacross the region (Fig. I of McHone 1981). sirnilar to NEQ (Cretaceous)mrlgm&s K-Ar and Rb-Sr whole-rockand mineral datesappear to cluster around 115, 125, and 135 Ma within the The original proposal for a Champlain Valley lobe across the Vermont - New track includedall of the White Mountainmagma series York border fMcHone & Corneille 1980). In the Q\fMMS) of New Hampshire(Morgan 1971), which northern Taconics region approximately 100 km aremainly Early Jurassicin age.The WMMS hassince farther south,the Cuttingsville complexand associated been describedas a province separatefrom the larger dikesconsistently give an agebetween 100 and 110Ma region of Cretaceousinfrusions (McHone & Butler (McHone & McHone 1993),whereas 50 km to the east 1984).Several Triassic plutons also are known, which of Cuttingsville, the Ascutney pluton and associated may themselves form an independent igneous dikes give an age near 122 Ma (Hubacher& Foland province.Although mainly granitic and syenitic rocks L99r). comprise the WMMS, many Early Jurassicalkaline dikes and several mafic to felsic plutons also in the Seamountvolcanism is not lirnited to province are petrographicallyand chemically similar hntspot-progres sion age s to Eady Cretaceousintrusions that overlap. Because the proposed trace of the New England hotspot Duncan (1984) compiled and measuredboth K-Ar commenced to the northwest of New Hampshire and Ar-Ar whole-rock and mineral datesfor dredged and continuedfar beyond it the WMMS plutons and and drilled samples from tops and flanks of New volcanic rocks cannotrepresent an early expressionof Englandseamounts. Most datesrange between 80 and the samemantle plume. 95 Ma along various seamountpositions of the chain, If we require a similax origin for similar'1esks,an except for a cluster of 4ArPeAr whole-rock dates earlier plume could be proposed that was coinci- around 101 Ma for the westernmost(Bear) CONSTRAINTSON MANTLE PLUME MODEL 329

'r30 potentialstretigraphic age rangeof Bearseamount = 120 ,e :! I lE !, tE 5 c0 zdb6 s (! 110 \ = s 8 g EE* N (, .E <€ t ta 100 E t-..- ----. o u c io (, 90 s *E I r (j E E* 9** a '-I-'- I .9 8E : E .st€ n G' 80 81,.N ftq-!il d. EB - proposedhotspot tnck/ I --1- 70 (vefocity4.7 cm/yr) E K-Ar age (bracket) B 60 406/39trr p, a sgs a:

0 200 400 600 800 10001200 1400 16001800 2000 2200 Distance(km) from westem MonteregianHills Ftc. 2. Radiomehicages along the proposedNew Englandcontinent--ocean hot-spot track. Modified from Figure 2 of Duncan (1984),with additionaldates from Eby (1984),McHone (1984), Foland et al. (1986)and Hubacher & Foland(1991). The stratigraphicallysstabli5hed minimum agefor the Bear seamountis given tn Swift et al. (1986).Older (Jurassicand Triassic) alkaline rock agesof the WMMS and New Englandaxe not paxtof this track and are onitted.

(Fig. 2). Using the Bear cluster and a tenuous was presentbefore l20M.a(Svrift et al. 1986)and that connection to the 70-75 Ma Corner seamounts(a the Mytilus seamountwas sufficiently active to form separategroup about400 km eastwardof the Nashville nn 6ssanisisland in mid-Tertiary time. seamount),Duncan (1984) proposeda linear progres- sion that alsofits most of the oldest4ArFeAr seamounr Nau Englandprovinces are not on line with the dates. This same line trends into the younger ages seamoants measuredfor the Monteregian Hillso as well as for some (but not all) of the New England infiusions The New England chain of seamountsis not very (Fig. 2). Duncan(1984, p. 9982) suggestedthat the linear. As is shown in Figure 1, the easiern section dates that do not flt this line, many of which were (Nashvilleto Gregg)trends toward southernmostNova obtainedby the K-Ar method, are unreliable or less Scotia- whereasthe westem section is on line with lmportant to the supporting evidence of an age southemmostNew England. Neither section trends progression through the continental province and toward the NEQ province of centralNew Englandand seamountchain. Quebec.The GeorgesBanks basalts(Hurtubise er a/. If one maintains an analogy with the continental 1987) could representa nearby continuation of the intrusions,the radiometricand geologicalevidence for seamounts,but an undatedpluton in the Gulf of Maine the seamountsalso suggeststhat their volcanic activity ("N" in Fig. 1; Klitgord et al. 1988) is the only was not limited to agesdefined by the linear hotspot- potentiallink in a conspicuousgap of Cretaceousrocks track. Rock samplesfrom the top or flanks of the betweenthe seamountsand southeastern New England. seainountsmay only date relatively minor - The segmenledpauern of seamountsis similal 1e cappingphases of igneousactivity. More importantly, patternsformed by smaller sectionsof the continental stratigraphicevidence indicates that the igneous rocks, such as the nortlwest-southeast 330

elongate pattern of the Monteregian Hills, and and cools away from ocean ridges and seamount westward-trendinglobes of dikes and plutons in activity. Crough(1981) proposed the sameeffect for westernNew England (McHone 1984). The Jurassic the New England area in Cretaceoustime, citing White Mountain magmaseries forms its own roughly especiallythe Adirondackdome to the westof the track north-south group in New Hampshire@g. 1). Dike as well as otherbasement highs to the northwestof the swarmswithin sectionsof the NEQ provincealso show track in Canada.Sleep (1990) utilized this conclusion distinctive linear groupsand frends,which McHone & as evidencefor changesin the'"buoyancyflux" ofthe Shake(1992) ascribedto struchualcontrols such as proposedmantle plume. Crough (1981, pers. comm.) fracturezones within the upper crust. Both continental expresseddoubts that a mantle plume is capableof and ocean-islandigneous complexes commonly show mechanically lifting the overhead lithosphere, nor stuctural orientationsof fracturesand dikes that have could it significantly heat the lithosphereby conduc- beenrepeated through time. For example,the Canary tion from below. Instead, hotspot swells could be Islands show distinct rifts and dike zones that have expectedto result from lithosphericexpansion by heat been active from the Late Cretaceousto the Dresent from magmasinjected within the lithosphere,thus an (Carracedo1994). indirect result of any mechanismfor magmatism. Topographic highs of the southem and northern Many other searnountsand continentalplutons Appalachians,and the Bermuda Rise of the western are not on a fuitspot trace Atlantic, contain Cenozoic alkaline igneous rocks (de Boer et al. 1988),but do not conforrn to tracesof It is also possible that the New England chain of any known hotspot (Vogt 1991).Various est''rates seamounts actually trends westward near the of Mesozoic and later uplift of central New England continent-oceanboundary (marked by the ECMA, havebeen based upon fission-tack studies@oherry & Fig. l) so that it connectswith the Great StoneDome Lyons 1980), statigraphy (Zen L991), and Ar-Ar and other geophysical4aemali$ (A-I, Fig. 1) that isotopes(West et al. 1993).Estimates of 3 to 8 km of probably represent Cretaceousplutons or volcanic erosionare common,especially near major high-angle plugs (Jansa& Pe-Piper 1988, Klitgord et al. 1988). faults.However, as well discussedby Znn (1991),the However, the continental margm plutons and geo- presenceof both Jurassic and Cretaceousvolcanic physicalanomalies (A-M, Fig. 1) essentiallyform their rocks in New England constrainsthe amountof uplift own northeasterlyline that trends toward the Early and erosionto no more than 2 to 3 krn for associated CretaceousScatarie Ridge dikes and flows off shore plutons on the hotspot track. Maps of formation from Nova Scotia(Fig. 1, Jansaet al. 1993).Similar boundaries and structures in central New England alkaline dikesand plutons occur farther to the northeast show no particular patternsof deforrnationthat would around Notre Dame Bay, Newfoundland(Fig. l; indicate greater uplift than in other sections of the Strong & Hanis 1974) and to the eastin the southern Appalachians, although small differences might be Grand Banks (Jansa& Pe-Piper l988,Pe-Piper et al. difficult to discern. 1994).To the southweslLate Jurassicalkaline plugs In supportof an uplift zone,Triassic-Jurassic sedi- and dikes are known in a limited area of western mentary basins are apparentlysmall or absentwithin Virginia (Fig. 1; Raderet al. 1986). the Gulf of Maine along the hotspot track (Klitgord The northeasterly trending continental margin et al. 1988).It is conceivablethat this conspicuousgap plutonic line roughly parallels a linear group of betweenthe Bay of Fundy and the Hartrord basin of kimberlites and kimberlitic intrusionsin the interior of southemNew EnglandGig. 1) is dueto deepererosion easternNorth America (Frg. 1). Although radiometric of early Mesozoic sediments.The generalscarcity of ages for the kimberlites vary, they could all be late Cretaceousplutons between New Englandand the New Jurassicin age, and Crough et al. (1980) and Crough England seamountsprovides no help to the model (1981)proposed that theseand otherkimberlites also of uplift by magmatismin this area.Other Cretaceous may mark Mesozoic mantle plumes. This and other plutons alongthe continentalmargin, such as the Great northeast-southwest linear groups :ne difficult to Stone Dome and Scatarie Ridge intrusions, are reconciie with plate motions, however, and a litho- associatedwith small domal uplifts (Jansa& Pe-Piper spheric fracture - tectonic control is more widely 1988), but not large anomalousswells or anticlines accepted(Panish & Lavin 1982, Jansa & Pe-Piper that might affect pat[ernsin nearby Mesozoic basins r988). (Frg.1).

Thereis little evidencefor uplffif'romheating Thereis no consistentchemical signature along a plumc track of a mantleplume

Crough(1978) noted that many volcanicregions are Geochemicalcharacteristics that are proposed to also topographichighs, or swells. This is especially mark plume-derived basalts include enrichment in evident on the oceanfloor, which subsidesas it ages volatiles, large-ion elements, light rare earths, and CONSTRAINTS ON MANTLE PLI,\4E MODEL JJI radiogenicSr-Nd-Pb isotopes @ilson 1993);many of Crustalstructures that extendedin responseto larger thesecharacteristics are consideredto be derivedfrom plate-tectonicevents are thereforea commonregional long-lasfing manile zones or reservoirs that underlie characteristic of alkaline igneous rocks in western the depleted upper mantle. Chemical enrichment of Africa, New England - Quebec,and easternCanadao mantlezones could alsoresult from migration of fluids although not to the degree of major rift-zones of (Nielson& Wilshire 1993),older magmas frozen in the eastern Africa and the western United States. mantle (Sleep 1984),or sectionsof subductedcrust Variations in chemistryin continentalrifts also (Pegram 1990). The depthsof such zonesare poorly have been related to lithospheric thinning along pre- constrained.A plume should contribute magmasor existing crustal structures, which produced mantle fluids that mix with shallow-mantlemelts. because the upwelling in the rift zone and decompressivemelting plumeitself mustmelt asit rises(Sleep 1984); its heat of the lower lithosphere and upper mantle @ailey is also transferredbetter by injecting its melts than by 1992.Bradshaw et al. 1993.Pederson & van der Beek much slower conductiontlrough adjacentsolid mantle 1994\. (Crough1978). The question continuesas to exactly what consti- Eby (1985)found that radiogenic Pb hadbeen added tutes a mantle plume, and of what characteristicsa to relatively depletedmantle before the generationof mantleplume mustimpart to alkalineand other basaltic alkaline New England- Quebeccontinental magmas, magmas.Studies of mantle xenoliths provide support possibly as a metasomaticevent. Among the New for ideas of lateral and vertical heterogeneityin the England seamounts,Taras & Hart (1987) found mantle, as well as for intramantle melts that change essentially random variations of trace elementsand composition with migration (Nielson & Wilshire isotopic characteristics,with no geographic trends 1993). A heterogeneoussuboceanic mantle (perhaps except for higher levels of radiogenic Pb toward the with zones of ancient plume-related material) can southeast.The involvementof severaldifferent mantle explain many of the variations found in ocean-basalt sourcesis likely, includinga less-radiogenicPb source chemistry (Sleep 1984). Many alkaline and tholeiitic for the New England - Quebecigneous rocks @by basaltsshow patternsof elementenrichment that were 1985,Taras & Hart 1987).Cretaceous alkaline rocks apparently influenced by ancient subducted crust ftom eastemCanada also have different trace-element within the upper mantle (Wittke et al. L989, Pegram and isotopic compositions(Pe-Piper et al. 1994), with 1990). The magmatic contributions of deep-mantle no consistentcompositional relationship to oneor more plumes, if any, must be maskedby melts from these mantle plumes. mantle heterogeneities.Finally, seismic tomography modelsof the uppermantle clearly showlarge domains DIScussIoN of high temperaturesthat cannot be tied to hotspot plumes(Anderson et q.l. 1992).A very wide domainof The Cameroonline of west-cenfal Africa presents high temperaturesmay be expectedfor magmatismthat the most likely analogy with the New England wasnearly simultaneousin the separatedigneous zones pluton - seamountline. The Cameroonline extends of northeasternNorth America (Ftg. 1). nearly 2000 km from central Africa soutlwestward, One or even several deep-mantleplumes do not crossing into the Gulf of Guinea (South Atlantic provide a satisfactory mechanismfor producing the Oceancrust). The line containsat least 17 volcanoes Cretaceousalkaline rocks of northeasternNorth and 60 continental plutonic complexes that show America and the adjacentwestern Atlantic. If altered igneous activity over a 65 Ma time span (latest compositions of mantle are reflected in alkaline Cretaceousthrough Cenozoic), but with no age basalts, perhaps additional samples of tlte ocean progression in the continental expressions of seamounts,continental plutons, and xenoliths in dikes magmatic activity (Moreau et al. 1987). The line will outline whereand how mantlezones of the proper includestlree oceanicislands that have all beenactive enrichmentare formed. If an extendedenriched zone within the last 5 Ma, but with their oldest rocks within the upper mantleis a commonlink for magmas decreasingin age from 31 Ma to 4.8 Ma oceanward for New England and the seamountchain, there must (I*e et al. 1994).Moreau et al. (L987) demonstrated also be other. smaller zones that have contributed a strong correlationof the Cameroonline with a zone magmasover much shorter time-spansinto true non- of lithospheric faults and other structures,consistent Eackhotspots in the continentand ocean crusts. In both with other conclusionsfor structural controls of cases,lithospheric tectonic actions were necessaryto magmatism in Africa @ailey 1992). Although Lee start and stop the generationof magmasfrom the same et al. (1994) proposedthat the three oceanicislands sourcesin the mantle.New petrologicalstudies should mark the track of a mantle plume, they describedthe emphasizethe local and regional tectonic featuresthat presenceof a "hot zone" of enriched sublithospheric could act in concertwith larger-scalerifting eventsand mantlethat producedsimilar continentalmagmas over movementsof plates around and beyond the Atlantic a long period, in responseto tectonic controls in the Ocean basin, as controls of the generation and Iithosphere. emplacementof alkaline magrnas. 332 TIIE CANADIAN MINERALOGIST

AcKNowLEDGEIVIENTS DEBoER, J.2., McHoNE, J.G., PUFBR, J.H., Recuro, P.C.& WHrrrn{GToN,D. (1988): Mesozoic and Cenozoic Continental Margin My interest in this topic was greatly stimulatedby magmatism.1r The Atlantic @.E. Sheridan& J.A. Grow, eds.).Geol, Soc. Am., TheGeology Thomas Crough, who challengedmy notions about of North Anertca I-2, 21,7-241,. events in the earth's mantle that are related to New England's igneousrocks. Other colleagueswho have (1980): discussed some of these problems include Robert DoHsnry.J.T. & Lvorvs,J.B. Mesozoicerosion rates in northernNew England.Geol. Soc. Am., Bull.9l,16-20. Butler, Jellede Boer, NelsonEby, KennethFoland, and Anthony Philpotts. They are not responsiblefor my misconceptions,nor do they necessarilyagree with DuNcar.r,R.A. (1984):Age-progtessive volcanism in the New my arguments.I thank Nelson Eby, Robert Duncano Englandseamounts and the openingofthe centralAtlattic Ocean. Geophys.Res. 89,9980-9990. Lubomir Jansa-and Robert Martin for their editorial "L suggestionsand improvements.Early portions of this work were supportedby Geochron Laboratories of Esv, G.N. (1984): Geochronologyof tle MonteregianHills province, Cambridge,Massachusetts and by National Science alkaline igneous Quebec.Geology 12,468470. FoundationGrant EAR7G14271. (1985): Sr and Pb isoopes, U and Th chemistryof ttre alkaline Monteregian and White Mountain igneous provinces,eastem North America, Geochim.Cosmachim. RSFERENcES Acta 49, 11,43-1153. ANDERSoN,D.L, (1994): Superplumesor supercontinents?. Geolngy22,3942. Epp,D. & Suoor, N.C. (1989): Distribution of seamountsin the North Atlantic. Nature 337, 254-257. TAN[\4oro,T. & ZnANc, Yu-Srmr (1992): PIate tectonics hotspots: third 256, Folem, K.A. & Feul, H. (1977): Ages of the White and the dimension.Science - 1il5-165t. Mountain intrusives New Hampshire, Vermont, and Maine,USA. An. J. 9ci.277,888-904.

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