619
The Canadian Mineralogist Vol. 37, pp.619-634 (1999)
OCCURRENCEOF EARLYCARBONIFEROUS HIGH'Zr RHYOLITES, COBEQUIDHIGHLANDS. NOVA SCOTIA: TEMPERATURE EFFECT OF A CONTEfiPONNruEOUSMAFIC MAGMAS
DAVID J.W. PIPERlNN GILLES DESSUREAU
Geological Suney of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada
CEORGIAPE-PIPER-
Department of Geology, Saint Mary's (Iniversity, Halifax, Nova Scotia B3H 3C3, Canada
ABSTRACT
The Fountain Lake Group of the Cobequid Highlands is a bimodal rhyolite-basalt volcanic unit that was extruded during the earliest Carboniferous along the southern margin of the Maritimes Basin in northern Nova Scotia. In the eastem Cobequid Highlands, up to 4 km of felsic lava flows and pyroclastic deposits are overlain by 1.5 km of continental tholeiitic basalt. A distinctive high-Zr rhyolite (>900 ppm Zr) occurs as ignimbrites near the top of the felsic volcanic sequence,as dykes cutting the main extrusive rhyolite succession (but not younger units), and as linear intrusive bodies along the northern margin of the Wentworth pluton. In the westem Cobequid Highlands, the volcanic successionis less than I km thick and comprises rhyolite with lesser basalt. Rhyolites have abundancesof trace elements (i.e., Zr, Nb, Y, Ga and Rb) typical of A-type, within-plate granitic magma. Older rhyolites have low ey6 (< +1), low GalAl and high Th/La, similar to early phasesofcoeval granite plutons in the region. Low-Zr rhyolites in the west show either a progressive increaseinZr with fractionation or no systematic pattern, whereasthose in the east show a slight decrease.Younger high-Zr rhyolites show a rapid increasein Zr with fractionation, and Zr is almost entirely in the groundmass.These high-Zr rhyolites are only slightly enriched in other high-field-strength elements and REE compared with the low-Zr rhyolites. Their Nd isotopic composition is close to that of contemporaneouslarge gabbroic plutons, suggestingthat the felsic magma may in part be derived from fractionation of mafic magma. The high Zr content is the result of high-temperatue suppressionof zircon crystallization by ambient mafic magmas correlative with the thick continental tholeiite basalts. Kelwords: rhyolite, Carboniferous, Nova Scotia, zirconium.
SovnrunB
Les roches du groupe de Fountain Lake, qui affleure sur le plateau de Cobequid, dans le nord de la Nouvelle-Ecosse,constitue une association bimodale rhyolite-basalte mise en place au d6but de I'bre carbonifbre le long de 1abordure sud du bassin des Maritimes. Dans la parlie orientale du plateau,jusqu'd 4 km de laves et de roches pyroclastiques felsiques sont recouvertespar 1.5 km de basaltethol6iitique d'affinit6 continentale.Une unit6 rhyolitique se distingue par sa teneur 6lev6e enZt (>900 ppmZr); elle se pr6senteen ignimbrites prbs du haut de la s6quencefelsique, en filons recoupant cette s6quence,mais non les roches plus jeunes, et en massifs intrusifs lin6aires le long de la bordure nord du pluton de Wentworth. Dans la partie occidentale du plateau, la stratigraphie des roches volcaniques est inf6rieure d 1 km, et les rhyolites sont prddominantes par rapport aux basaltes.Les rhyolites possddentdes teneursen 616mentstraces (i.e., Zr, Nb, Y, Ga et Rb) typiques de magmas granitiques de type A, form6s dans un contexte inEa-plaques.Les rhyolites plus anciennespossbdent une faible valeur de eNa(< +1) et de Ga/Al, et une valeur 'IhILa 61ev6e,ressemblant ainsi aux phasespr6coces des plutons granitiques contemporains de la r6gion Les rhyolites d faible teneur en Zr dans I'ouest montrent soit une augmentationprogressive en Zr due au fractionnement ou bien un comportement non syst6matique du Zr Celles du secteur oriental font preuve d'un l6ger enrichissement. Les rhyolites d Zr 6lev6, plus jeunes. monffent un effichissement marqt€ etZr avec\e fractionnementprogressif du magma, et le Zr se ffouve presqu'entibrementdans la matrice Ces rhyolites enrichies enZr nele sont que l6gdrement dans le cas des auEes6l6ments h champ 6lectrostatique61ev6 et les terres rares, par rapport au rhyolites h faible teneur en Zr. Leur composition isotopique en Nd ressemblea ce[e de plutons contemporains de composition gabbroique, ce qui fait penser que le magma felsique pourrait 6tre issu du fractionnement d'un magma mafique. La teneur 6lev6e en Zr r6sulterait de la suppressiondu zircon d temp6rature 61ev6ed cause de la pr6sencede magma mafique nourissier des 6paissescoul6es de basalte thol6iitique (Traduit par la R6daction) Mots-clds: rhvolite. carbonifbre. Nouvelle-Ecosse. zirconium.
s Geological Survey of Canadacontribution number 1998215. * E-mail address: [email protected]
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INrnooucnou & Peterson 1998), and younger rocks, such as the Visean-Westphalian volcanic rocks of New Brunswick The mid-Devonian to lower Permian Maritimes Ba- (Fyffe & Barr 1986), have thinner rhyolite sequences sin (Fig. 1) is a major post-tectonic extensionalbasin rn and a higher proportion of basalt to rhyolite. The thick Atlantic Canada (Bradley 1982). Most of the volcanic sequencesof felsic volcanic rocks in the Cobequid rocks in the basin are continental tholeiitic basalts Highlands are thus regionally unusual. (Blanchard et al. 1984, Pe-Piper & Piper 1998). How- The thick rhyolite sequencesin the Cobequid High- ever, extrusive and hypabyssal rhyolites also are wide- lands show considerable geochemical variation. Both spreadalong the Cobequid crust-scaleshear zone at the whole-rock geochemistry (Pe-Piperet aL 1991) and Pb southern margin of the basin, where the Fountain Lake and Nd isotopic data (Pe-Piper & Piper 1998) of coeval Group consistslocally of as much as 3-3.5 km of felsic granites suggest that the felsic magmas result from volcanic rocks overlain by 1-1.5 km of basalt. The anatexis in the lower crust, with heat derived from co- Fountain Lake Group outcrops in several areas of the existing mafic magma. The chronology of emplacement Cobequid Highlands (Fig. 2), where ithas beendismem- of magma pulsesis well known from volcanic stratigra- bered by later faulting. Palynology (Utting et al. 1989) phy, cross-cutting relationships and radiometric dating. indicates a Toumaisian age for at least the upper part of In this paper, we attempt to constrain i) the petrogenetic the Fountain Lake Group, and preliminary U-Pb zircon history of the felsic magma, ii) the evolution of distinc- dating (Dunning et al. 1997) indicates that the felsic tiveZr-richphases, and iii) the relationship between the volcanic rocks are mostly 358-354 Ma and thus of ear- felsic and mafic flows of the Fountain Lake Group. We liest Carboniferous age according to the time scale of establish a comparison with the minor rhyolite se- Tucker et al. (1998). These volcanic rocks are thus quenceselsewhere in the Maritimes Basin. somewhat younger than the main granite and gabbro plutons of the Cobequid Highlands (360-363 Ma: Doig Gpor-ocrcel Serrwc et al. 1996). Within the Maritimes Basin, both older volcanic successions,such as the late Devonian Fisset The Cobequid Highlands lie on the southernmargin Brook Formation of western Cape Breton Island (Barr of the Late Paleozoic Maritimes Basin (Fig. 1). This
Ftc. 1. The Maritimes Basin, Atlantic Canada,showing location of the Cobequid Highlands and Devonian-Carboniferous volcanic rocks
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s'N Squally-. PoinKo
.zl . ilt i North River DEVONIAN.CARBONIFEROUS Pluton -45'^ \ zoN A FountainLake Group E Carboniferousand \ c.p" E Maficplutons Triassicsedimentarv mcks Chignecto E Felsic plutons + well s.w Plulon
Frc. 2. Devonian-Carboniferous igneous rocks of the Cobequid Highlands, Nova Scotia. Map modified from Donohoe & Wallace (1982) Numbers refer to localities in Figure 3.
basin accumulated several kilometers of middle Devo- (1989, 1991) demonstratedthat the rhyolites of the nian to lower Permian strata, following the early to Fountain Lake Group were geochemically similar to the middle Devonian Acadian orogeny (Bradley 1982). early Carboniferous granitic plutons of the Cobequid Widespread igneous activity accompaniedbasin forma- Highlands. These plutons are subalkaline, and their en- tion, which resulted in the emplacementof several ma- richment in Nb and Y with elevated Ga/Al ratios sug- fic and felsic intrusions, as well as the deposition of geststhat they are A-type granites:Pe-Piperet al. (1991) cogenetic volcanic suites in the Cobequid Highlands. interpreted the felsic magmas as produced by anatexis The southern margin of the Cobequid Highlands is of a halogen-rich anhydrouslower crust. The main gab- marked by the Cobequid-Chedabucto shear zone, which bro body of the Wentworth pluton (Frg. 2) shows wide- is a dextral strike-slip fault zone separatingthe Avalon spreadevidence for coexisting mafic and felsic magmas terrane from the Meguma terrane to the south (Keppie (Pe-Piperet al. 1996).Pe-Piper & Piper (1998) reported 1982). The northern margin of the highlands is Pb and Nd isotope determinationsfor both granites and unconformably overlain by Carboniferous sedimentary felsic volcanic and subvolcanic rocks of the Fountain rocks. Lake Group. In the eastern Cobequid Highlands, the Fountain Lake Group is divided into two formations, the Byers Frpr-oOccunnrNcE oF TIIEFelstc Rocrs Brook and Diamond Brook formations (Donohoe & OFTHE FOUNTAIN LAKE GROUP Wallace 1982). The earliest volcanic rocks consist prin- cipally of rhyolite (Byers Brook Formation) with minor The occurrenceof felsic rocks of the Fountain Lake interbeddedbasalt (Fig. 3). Theseare overlain by basalts Group is described from west to east through the of the Diamond Brook Formation, locally with small Cobequid Highlands (Figs. 2, 3). At northern Cape rhyolite domes (Dessureauet al., in press). The Byers Chignecto, severalfault-bound slicespreserve segments Brook Formation is from 1 km to >3 km thick in the of a rhyolite - basalt sequence(Piper et al. 1996a).Most Earltown - Byers Lake belt of the eastern Cobequid of the rhyolites are porphyritic and probably intrusive, Highlands (Fig. 2), where it outcropsin a seriesof north- but at Squally Point (location 1, Figs. 2, 3), an 80-m- flowing brooks and dips 75o-80oN. The western part of thick volcanic sequenceis exposed,including thin ba- this area was studied in detail by Gower (1988). In the salt and a 35-m-thick ignimbrite. Scotsburn anticline to the east, the Irving Chevron In the West Moose River area,the volcanic sequence Scotsbum #2 well (Utting et al. 1989) intersected 1.5 lies immediately north of the Cobequid Fault, dipping km stratigraphic thickness of predominantly felsic vol- steeply near the fault and flattening out to the nor1h. On canic rocks. In the western Cobequid Highlands, the the West Moose River road (5 in Figs. 2, 3), the section Fountain Lake Group is less than I km thick and com- consists of 500 m of rhyolite flows with minor pyro- prises rhyolites and felsic pyroclastic rocks interbedded clastic flows overlain by alternating basalt and rhyolitic with lesser basalt flows (Fig. 3; Prper et al. 1996a). pyroclastic flows, whereas farther west, in McAlese On the basis of a reconnaissancestudy, Dostal et al. Brook (4 in Figs. 2,3),basalt is absent. (1983a) concluded that the rhyolites of the Fountain Thin (< 600 m) east-west-striking belts of the Foun- Lake Group formed in an intraplate setting by crustal tain Lake Group outcrop in fault-bound sections ex- anatexis,and U, Th and K were remobilized during sec- posed on the New Britain Road and in Economy River ondary processes(Dostal et al. 1983b). Pe-Piper et al. (6,'7 rn Figs. 2, 3), north of the North River and Pleas-
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l3.SCOTSBURN EAST WEST ANTICLINE
CHIGNECTO EARLTOWN-BYERS LAKE BELT
WESTMOOSE RIVERAREA ByersBrook 11 Diamond 5 West Moose Brook River Road 9.Swan Formation 4 Brook 1000m 8192: 3 Seal Cove 4908 /-'--.1 8191: 4877 H,.;l I: Byers 4248 ffi ? Brook 4204 ffifi Formation F*l ti-l-l
Porphyritic o) rhyolite 0 o o Pyroclastic = 2638m 500m Rhyolite flows ano oomes Conglomerate and sandstone 1000m Undivided rhyolite
4)
o o)=
FIc. 3. Summary stratigraphic columns of the main developments of the felsic Fountain Lake Group in the Cobequid Highlands. Columns located on Figure 2; (9) from Gower (1988), and (13) from Uuing et al. (1989).
ant Hills plutons. These outcrops dominantly contain Pyroclastic flows and afu-fall tuffs of rhyolitic compo- rhyolite and pyroclastic flows interbedded with minor sition are the dominant lithology of the Fountain Lake amounts ofbasalt and minor hypabyssalintrusive rhyo- Group in the Collingwood area (8 in Figs. 2, 3). These lite. Steep south-dipping thrusts separatethe Fountain units achieve thicknessesof several hundred meters rn Lake Group from Neoproterozoic and Silurian sedimen- steeply dipping units in Bulmer and Arsenic brooks. tary rocks to the south, and gabbro and minor granite of Minor rhyolite and basalt associatedwith these units the late Devonian Wyvern Pluton to the north (Fig. 3). occur as flows and hypabyssal intrusions.
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The largest occurrence of the Fountain Lake Group TABLE I ANALYZED SAMPI,ESUSED IN THIS STTJDY lies between Earltown and Byers Lake. In this area,the rocks are steeply dipping and can be divided into two Smple Loetion Loqtion UniverealTwffi Rwk Nmbq Mq€torcoordinstes Type mappable formations. The predominantly felsic rocks of the Byers Brook Formation are between 1 and 3.5 4204 SealCove 8701805 3497678-5030709N Intrusion km thick and are conformably overlain by approxi- 4235 SealCove 8711510 349U6E-5O29648N Inttusion mately 1.5 km of predominantly mafic rocks of the Dia- 4248 S@lCoYe 8'l1l5l2 349267E-5o29464N lntrusion mond Brook Formation. Gower (1988) investigated the 4877 SealCow 9000304 349341E-5029390N Intrusion 4908 Ssl Cove 9000409 349228-5029762N Dyk€ stratigraphy in detail between Wentworth and Byers 49a7 SquallyPoint 9000515 350678E-5032474N Flow Lake, and used information from a large number of dia- 49Et SquallyPoint 9000516 350685E-5032507N Flow 4995 SquallyPoirt 9000519 350614E-5032517N Tuff mond drill holes together with field mapping. In this 4996 SquallyPoint 9000519 350614E-5032517N Flow area, he recognized two discrete volcanic cycles sepa- 4997 SquallyPoitrt 9000519 350614E-5032517N Flow rated by a conglomerate-siltstone horizon. Ash flows 5000 SquallyPoint 9000523 350399E-5032536N Flow 5014* SqualyPoint 900060E 350454E-5032530N lSnimbtite and tuffs are dominant in the lower portion of each 501E SquallyPoitrt 9000616 3502018-5032275N Flow cycle, passing upward into rhyolite flows and domes 5O2O SquallyPoint 9000620 350262E-5032075N Flow 5075 SqualyPoiDt 9101205 350550E-5032550N T\rff interbedded with basalt and some tuffs (9 in Fig. 3). 5083 SquallyPoint 9101209 350256E-5032238N Flou Prominent ignimbrites outcrop in Swan Brook (10 in 7592 SquailyPoint 9000620 3502628-5O32075N Flow 7597*t CpeObigrec,o 9404601 351l01E-5023291N Dykein Fig. 2) near the top of the Byers Brook Formation. Far- pluton ther west in Whirley Brook (11 in Figs. 2 and 3), the 40'12 W MooseRivd 8623107 406435E-5033967N Flow thicknessofpyroclastic rocks seemsmuch reduced,and 4073 W MooreRivq 8623110 4063778-5034098N Flow 4074 W Moow Rivs 8623111 406367E-5034102N Flow only a few hundred meters of the lower cycle are 4075 W MooseRivq 8623114 406343E-5034302N Flow present. In the eastem part of the Earltown - Byers Lake 4077 W MooseRivq 8623116 406329E-503437EN Flow Flow (12 40'78 W Moosefuver E623118 406325E-5034552N belt, rhyolite domes predominate in Fig. 3), and the 40t0r W MoowRivd 8623120 406350E-5034627N Flow overall thickness is as little as a kilometer. Outcrop is 4109 W M@sRivq 8623310 406491E-5035006N Flow quite discontinuous in both sections ll and 12, and the 4tl5 W MoosRivq 8623325 406513E-5035587N Igoinbrit€ 4116 \{ MooseRivq 8623327 406481E-50357i8N ?Intrusion logs presentedmay overestimatethickness of resistant 4l 1E W MooseRivq 862332E 406367E-5035875N Flow rhyolite flows and domes and underestimatethickness 7330 BlrckBrook 9403210 430632E-5037369N silt 7339 BlrckBrook 03244 431687E-5038006N sill of pyroclastic units. The thinning from west to east in 60744 \{hirley Brook 9300332 470852E-5049494N Flow, uppq the Earltown - Byers Lake belt seemsto be achieved in BBFm 3 60831 Whirl€yBrcok 930041I 410a328-5M9244N Dyke in part by a reduction in thickness ofpyroclastic rocks and BBFn 3 in part by the absenceof the lower part of the formation 60871 Whirley Brook 9300426 470920E-5048859N Dyke in in the east. This absencecannot entirely be accounted BBFn 4 63071 WhirleyBrook 9302607 470671E-5048220N Nnrgin for by intrusion of late plutonic rocks into the base of W pluton 4 the formation, and must result either from non-deposi- 6688t WB, Northtuvq 9402116 475178E-5045434N N mugin plutoo tion or removal of the base of the faulting W 4 section by 6682 W B North Rivq 9402108 4781048-5056855N Flow, middle The Fountain Lake Group is also recognized from BBFn 3 seismic reflection profiling in the subsurface of the 691st ByqsBrcok 15145 467804E-5050926N Dyke, bassl DBFn 3 Scotsbum Anticline and the southeasternCumberland 7589 Whirley Bmk 9300404 4705t68-505045tN Flow, middle basin (Durling 1996). Lithologies in the Irving Chevron DBFn 3 7290+t W B NorthRivq 9402717 4783l IE-50,K846N Nmtrgin Scotsbum#2 well are known only from cuttings and log W pluton 3 interpretation. 72911 WB NorthRivs 9402722 4783778-504494TN N mugin Rhyolite sills and dykes in places cut the Fountarn W pluton 3 736A WB NorthRivd 9403539 4796468-5046639N ?Domg lows Lake Group. Their spatial relationship to the Fountarn BBFn 3 Lake Group and the coeval plutons suggeststhat they 7369 WB NorthRivs 9403545 4798028-5045695N ?Dom€,low BBFm 4 are chronologically and genetically related to the volca- 7311 WB NorthRivq 9403549 480602E-5044581N ?Dome,basal nism. Most appear to be relatively late in the volcanic BBFm 4 7704t WhirleyBrcok 9505020 47r2488-s047455N N margid sequence.At Seal Cove, volcanic rocks are cut by a W pluton 3 rhyolite dyke (sample 4908; Table l). A rhyolite dyke 7714 Bym Brook 9505313 469034E-5049188N ?Dm,lows (7597) cuts plutonic rocks of the Cape Chignecto plu- BBFn 3 8l9lt SwmBrcok 9502816 461i85E-505 1970N Ignimbrite 3 ton that show a syn-intrusion deformation (Koukouvelas 81921 SweBrook 9502t21 ,t60997E-5051502N Igninbrite 3 et al. 1996). North of the Pleasant Hills pluton, in the 8195 SweBrook 9700319 4607158-5051l53N ?Intrusion 3 Black Brook region (Piper et al. 1996b), thrust faults within Silurian sedimentaryrocks are occupied by por- Intwim: irtepreted c a hypebysalirlrusire body. ?Intruion: ould be hypabysssr phyritic rhyolite sills (7330, 7339) that are geochemi- intnrsive body or dome. DBFm: Di@ond Brook Fomtioru BBFm: Byqs Brmk Fomtioq W plutm; Wqtworth Pfuton cally similar to Fountain Lake Group rhyolites * Nd isotopes(PePipq & Pip€r 1998). elsewhere. *+ Srcs of geoohmical mnlyss: I : Pipq et al (1996a),z Ptpq et ql. (1996b); papq, 4: dffi A seriesof rhyolite dykes with >900 ppm Zr (Gower 3: This Unpublished I Zr>900ppm 1988) cut the Byers Brook Formation (6083) and possi-
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bly the basal part of the Diamond Brook Formation is an accessorymineral in most samples,and a few con- (6915). In places,these dykes are composite diabaseand tain trace amounts of apatite, allanite or titanite. rhyolite (Gower 1988), with the diabasegeochemically Unwelded tuffs contain mostly felsic lithic frag- similar to the basalts of the Fountain Lake Group. ments, angular devitrifred shards,and quartz and feld- The basal part of the Byers Brook Formation is spar crystals. The matrix is microcrystalline and has poorly exposed. The formation appears to be fault- been chloritized and partly transformed to white mica. bounded and, in most areas,this fault acted as a path- Ignimbrites contain mostly angular devitrified shards, way for a series of steeply dipping sheetsof intrusive with a lower proportion of lithic fragments and crystals. rhyolite (6307, 6688, 7 290, 729 l, 77 O4), eachtypically The silicified pyroclastic rocks have a fine-grained a few tens of meters wide. Irregular enclavesand sheets groundmasswith zonesof very fine-grained quartz crys- of diabase occur within these rhyolites, with textures tals. The angular pyroclastic fragments consist mostly suggesting that the mafic magma was immiscible (cl of quartz, although some fragments are quartz with gra- Pe-Piper et al. 1996). These intrusive rhyolites separate nophyric intergrowths of feldspar. Accessory minerals the Byers Brook Formation from the Wentworth pluton include pyrite and allanite, with epidote and chlorite as granites to the south [the Hart Lake - Byers Lake gran- alteration products in the groundmass.Clast sizesin the ite of Donohoe & Wallace (1982)1.This intrusive rhyo- pyroclastic rocks vary from one sampleto another,with Iite unit is marked by a sffong magnetic anomaly (Piper most being lapilli tuffs. et al. 1993), suggesting the presence of voluminous The high-Zr rhyolite dykes and the intrusive high- mafic rocks at shallow depth. Zr rhyolites at the northern margin of the Wentworth pluton are porphyritic with quartz and feldspar pheno- Pprnocnapuy eno GnocueursrRy oF FBr-srcRocrs crysts. Titanite is found in all these rocks, and allanite op tnB FouNrarN LAKE GRoup and magnetite are coflrmon accessoryminerals in most samples. Epidote and chlorite are conlmon secondary Petrography minerals. Allanite, epidote and titanite commonly oc- cur as clusters of crystals in the groundmass,which is The Fountain Lake Group consists of several felsic interpreted as devitrified glass, and theseminerals may lithologies including rhyolitic flows, intrusive porphy- locally account for severalpercent of the modal miner- ritic rhyolite and a wide variety of rhyolitic pyroclastic alogy. In some samples,accessory pseudomorphs after flows including unwelded tuff and ignimbrite. Some of amphibole, with a characteristic l24o basal section, the samplesof rhyolite and pyroclastic rocks have un- comprise fine-grained quartz, white mica, chlorite and dergone silicification, resulting in rocks with up to 857o titanite. Zircon is not seen as an accessory mineral. silica. Microprobe X-ray mapping shows that zirconium oc- Most of the samplesof intrusive porphyritic rhyolite curs only in 1-4 pr"mcrystallites in the groundmass are red, purple or grey and have a bimodal distribution (Fig. a). The dyke from the Cape Chignecto pluton has of grain size in the groundmass. The coarse-grained a spherulitic texture in the groundmass. zones are mostly quaftz with some altered plagioclase, whereas the formerly glassy zones are cloudy and al- Geochemistry tered to chlorite and white mica. Phenocrystsoccur in both zones and are mostly white to buff feldspar of The felsic rocks range in composition from dacite,
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80 ':i!- + a) r+ + tt Rhyolite + Fr+ t',^ 17 + ll,Vv
S 270 Rhyodacite-Dacite o a
b)
Comendite Panteilerite l o1 a x ^' Rhyolite .> A N Y Trachyte 0.1 Rhyodacite/Dacite . I TrachyAnd
0.1 NbA/
o N margin Wentworth pluton - r Earltown-Byers Lake Ftc. 4 (a) Back-scattered-elechonscanning image and (b) o a Swan Brook > Black Brook X-ray map for Zr of high-Zrrhyolite sample 6083, show- ^ West l\,4ooseRiver ing occunenceof zirconas small crystallites. v v Chignectopeninsula Open symbols indicate Zr > 900 ppm + Gower low Zr rhyolite x Gower high zr rhyolite Brook, as dykes cutting the Byers Brook Formation and possibly the basal Diamond Brook Formation in the Frc. 5. Discriminationdiagrams for rhyolite classification - Earltown Byers Lake belt, as a dyke in the Cape showingour bulkcompositions and those ofGower (1988) Chignecto pluton, and as intrusive bodies between the a) ZrlTiO2*l}a varszsNb/Y, after Winchester& Floyd base of the Byers Brook Formation and the main (1977)(no datafrom Gowerbecause levels of Nb andY Wentworth pluton granite. werenot documented);b) ZrtliO2+ l}a versusSiO2, after The high-Zr sampleshave silica values from -857o Winchester& Floyd(19'1'7)."High-Zr" refersto rockswith to as low as677o.The low-silica,high-zirconiumrhyo- >900ppm Zr lites generally have high TiOz, FeOt, P2O5, and Zn. Compared with low-Zr samples,they also have rather lower Al2O3 (Fig. 6a) and total alkalis (Fig. 6c) for the same SiO2 content and show slightly higher Y and M Lake belt show a slight decreaseinZr with increasing (Figs. 6e, f). In general,Zr shows a positive correlation Nb and Y. The high-Zr samplesshow a strong positive with the high-field-strength elements Nb and Y correlation of TiOz withzr (Fig. 7b), whereasthe low- (Fig.7a), although samplesfrom the Earltown - Byers Zr samplesshow no systematic trend.
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17
15
s \z *rs Y 3 e_ + u- o' 11 o o L z
;r 60 60 sio2 sio2 sio2
150
^ 100 E E E &rooo l"a ; ta.f-t N zso o r+t' 't it .v.'v a l
0L 0L 60 60 60 SiO z sio2 sio2
Ftc. 6. SelectedHarker diagrams showing element variation with SiO2: a) Al2O3, @) Fe2O35,c) Na2O + K2O, d) Zr, e) Y, and f) Nb. Symbols as in Figure 5.
E c $ rooo .&rooo
N N
Nb (ppm) 400
300 ox x x& e o E 20o rooo z E o& N 100
10 20 30 40 50 10 20 30 40 50 Th (ppm) Th (ppm)
FIc. 7. Selectedelementplots: (a)Zr versusNb, (b)Zr versusTiO2, (c) Nb + Y verszs Th, (d)Zr versusTh. Symbols as in Figure 5. Comparisons with the Fisset Brook Formation (FBFm) basedonBarr et al. (1996), Barr & Peterson(1998) and our unpublished data. Data for the Pleasant Hills pluton (PHP) based on Pe-Piper & Piper (1996). EBLB: Earltown - Byers Lake belt; WMRR: West Moose River road section.
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Thorium is commonly used as an index of fraction- Drscussrou ation in felsic rocks, although it may be mobile in some rhyolites of the Fountain Lake Group (Dostal er a/. Geochemical variation in eruptive products 1983b).Elements such as Rb, Nb and Y show a general in time and space increasein abundancewith increasingTh (e.g., Fig. 7c), the correlation being stronger when individual geo- In the easternCobequid Highlands, there is a clear graphic areas are considered. A plot of Zr againstTh stratigraphicdistinction between the felsic Byers Brook (Fig. 7d), however, shows a positive correlation for Formation and the mafic Diamond Brook Formation; some groups (e.g., the lower part of the West Moose the total thickness of volcanic products is everywhere River road section and most high-Zr rocks), but a slight greater than 2 km. In contrast, in the central and west- negative correlation in low-Zr rhyolites of the Earltown ern Cobequid Highlands, the overall stratigraphicthick- - Byers Lake belt (as was noted in Fig. 7a). Rhyolites nessis less than I km, and basalt units severalhundred from Squally Point show no clear pattern. meters thick are interbedded with predominantly rhyo- The high-Zr rhyolites have higher contents of the litic successions(cf rare basalts <100 m thick in the rare-earth-elements(REE) and a smaller Eu anomaly Byers Brook Formation). This abrupt diminution rn than the low-Zr rhyolites (Fig. 9). Low-Zr rhyolites from thicknessand stratigraphicinterfingering cannotreadily Squally Point show decreasingabundances of the light be explained by tectonic dismemberment or erosion of REE with increasing fractionation (measuredby SiO2, a succession originally similar to that in the eastern Th or Nb) but increasing abundanceof the heavy REE. Cobequid Highlands. We suggest that the formational In the low-Zr rhyolites of the eastern Cobequid High- lands, both light and heavy REE increasewith increas- ing fractionation, whereasEu decreases. Stratigraphic trends in geochemistry can be most readily assessedin the West Moose River road section, where a thick succession of rhyolite flows outcrops o (Fig. 3). Flows between 100 and 400 m from the base L of the succession show an upward decrease in most E large-ion lithophile and high-field-strengthelements and a concomitantincrease in Na and Sr, with an almost con- o 100 stantZrlTh value (Fig. 10). At Squally Point, in con- () trast, where eight samples are available through an o 80-m-thick stratigraphicsection (Piper et al.1996a, their E Fig. 7), no systematicvertical changesin element abun- o dance are recosnized. U)
LaCe Nd SmEu Tb Yb Lu 2000 on 1000 1000 oAo )r q) E L -f^'fi^.vV o "! o L '100 N vvV o vf ^Yr o ^ I q) I,S,and. | o-typ" M-typegranite l^ granite E (6 100 @ 110 10 (Ga/Al)x104 LaCe Nd SmEu Tb Yb Lu
FIc. 8. Plots of Zr versus Ga./Al for rhyolites of the Fountarn Lake Group. Rectangular box separatesA-type granites Frc. 9. Rare-earth-elementplots for the Fountain Lake Group (upper right) from I-, S-, and M+ype granites (after Whalen rhyolites normalized to C1 chondrite; (a) low-Z rhyolites, et al. 1987). Symbols as in Figure 5. (b) high-Zr rhyolites. Symbols as in Figure 5.
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Distancealong roadfrom nomenclatureof the easternCobequid Highlands should base of section(m) not be applied to the central and western highlands' The correlates spatially with a 100 200 300 400 500 600 700 change in volcanic style 0 changefrom the large Wentworth pluton in the eastto a series of small, more granitic plutons in the west. The 1 thickest succession of rhyolites [between Wentworth NarO z and Byers Lake, (9) in Fig. 3l is closest to the volumi- (wt%) nous heat-sourceof the gabbroic part of the Wentworth 3 pluton (Fig. 2). The distinctive high-Zr rhyolite ignimbritic flows occur near the top of the Byers Brook Formation in Swan Brook. The geochemically correlative dykes cut 'l only the Byers Brook Formation and possibly the ex- treme base of the Diamond Brook Formation, but do
J not cut younger rocks. This suggeststhat high-Zr dykes and pyroclastic flows are synchronous.The geochemi- K,O-5 _ similar intrusions at the northern margin of the (wt%) cally Wentworth pluton are probably also synchronous.This 7 high-Zr magma immediately preceded the voluminous flood basalts of the Diamond Brook Formation. The 0 only other geographic region in which high-Zr rhyolite has been found is a single dyke in the Cape Chignecto mafic dykes, proh- 100 pluton. In this area, large tholeiitic ably correlative with the Diamond Brook Formation, are Rb (ppml widespread in both the pluton and the adjacent Foun- 200 tain Lake Group.
Comparative geochemistry of the low-Zr rhyolites
The low-Zr rhyolites of the Fountain Lake Group are geochemically similar to coeval granites from the CobequidHighlands (Pe-Piper et a\.1991, Pe-Piper& Piper 1998) and to rhyolites from the Fisset Brook For- mation of western Cape Breton Island (Ban et al. 1996, Pe-Piper& Piper 1998,Barr & Peterson1998). In gen- eral, all these felsic rocks resemble A-type granites. Most sampleshavehighZr, Nb and Ga/Al ratios typical of A-type granites (Figs. 6d, f, 8), although some samplestrend into the I-, S-, or M-type granil.efreld (cf. Whalen et al. 198'7).The ratio Nb/Y shows no correla- tion with Zr content,and all sampleshave Nb/Y values below 1.2, thus corresponding to the ,A'2type rhyolite and granite in the classification of Bby (1990,1992), which are interpreted as resulting from crustal anatexrs. Most samplesplot as within-plate granites on the basis of trace elements such as Nb, Y, Rb on Pearce et aI. (1984) diagrams.These similarities imply that the petro- genesis of the low-Zr rhyolites is similar to that previ- ously inferred for Cobequid Highlands granites (Pe-Piperet al. 1991,1998, Pe-Piper& Piper 1998). Felsic magmas appear to be derived by melting of ma- terial in the lower crust as a result of the thermal effect of voluminous mafic magma. High GalAl and the com- mon presence of fluorite suggest that the source area was relatively anhydrous and rich in halogens. In detail, there are regional variations, most notice- Frc. 10. Vertical variation in abundanceof selectedelements able in the relatively stableHFS elements.Most granite in rhyolite flows exposed on the West Moose River road. samplestaken from plutons from the western Cobequid
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Highlands consisting of <10Vo gabbro have Zr < 300 felsic rocks of the Fisset Brook Formation have unusu- ppm. There is a much greaterrange of Zr contentsin the ally low Nb/Zr values compared with other volcanic Cape Chignecto (<800 ppm), PleasantHills (<850 ppm) rocks of the Maritimes Basin (Pe-Piper & Piper 1998, and Wentworth (<1000 ppm) plutons, all of which have their Fig. 12), which again may reflect differences in sizeablegabbro bodies. This range is a little greaterthan parental magmas. that in the low-Zr rhyolites of the Fountain Lake Group. Most differences in the abundanceof ftace elements Most rhyolites of the FissetBrook Formation have <200 in felsic rocks, however, develop during magma differ- ppm Zx, but higher NblZr, Th/Zr and TiO2/Zr values entiation(e.9., Evans & Hanson 1993).The ratio ofal- than both the Fountain Lake Group and the pleasant kalies to Si and Al has an important influence on zircon Hills pluton, although Y/Zr values are similar (Fig. 7). saturation.Watson & Harrison (1983) expressedthis as The systematicbehavior ofelement abundancesas a the cation ratio (Na+K+2Ca)/(Al.Si) and showed that function of Th, Nb and SiO2contents suggeststhat gen- zircon solubility dependson this cation ratio and tem- eral differentiation trends can be inferred from Harker perature. Plots of this cation ratio against Zr for the diagrams(Fig. 6). Thesetrends imply that feldspar frac- Fountain Lake Group (Fig. 11) show a weak positive tionation has predominated, with some removal of Fe, correlation for the low-Zr rhyolites. A similar weak Mg and HFS elementsby biotite and iron-titanium ox- positive correlation is shown by the PleasantHills plu- ides. Abundance of Zr and the REE is influenced by ton. The rhyolites of the easternCobequid Highlands, fractionation of accessoryphases. In the West Moose together with those of the Fisset Brook Formation and River road section, Zr and other HFS elementsincrease the Pleasant Hills granites, have a higher cation ratio with increasingTh. At Squally Point, Nb and y increase, for the sameZr content than do rhyolites of the western but Zr shows scatter.In contrast,in the low-Zr rhyolites Cobequid Highlands, demonstrating that alkalinity of the Earltown - Byers Lake belt (and also in the Fisset variations are not sufficient to explain observed varia- Brook Formation), Zr content decreasesslightly with tion inZr content. increasing Th, whereas the level of both Nb and y in- In metaluminous melts, fluorine increasesthe solu- creases,which suggeststwo different patterns of frac- bility of zircon (Collins et al. 1982, Keppler 1993). tionation of Zr in the low-Zr rhyolites. In the Earltown Recent experimental work by Mar' et a/. (1998) on - Byers Lake belt and at Squally Point, zircon is an ac- peralkaline melts showed that Cl substantially reduces cessoryphase, so that Zr is removed from the magma as zircon solubility. In the eastern Cobequid Highlanos, it differentiates. In the West Moose River road siction, Gower (1988) found no correlationbetween F andZr zircon is much less cofitmon as an accessoryphase, and contentsin rocks of the Fountain Lake Group. Our data Zr appearsto increase with differentiation. The lieht on basalt and rhyolite from throughout the Fountain REEdepletion with increasingdifferentiation at Squaily Lake Group and of associateddiabase and rhyolite Point is probably the result of removal of accessory dykes (resultsof40 analyses)and from the FissetBrook allanite (c/. Evans & Hanson 1993). Such trends can be used ro interpret stratigraphic variation in extrusive rhyolites. In the basal 400 m of the West Moose River road section, most fractionated magmas (low levels of 5 Na, Ca, Sr; enriched in larse-ion a lithophile elementsconcentrated in hydrous mettJ; frigtr Ga,/Al)were eruptedfirst, followed by progressivelyless FBFm fractionated magmas. Such a sequenceis analogous to that produced by progressiveeruption of magmas from O ire a zoned chamber (Hildreth 1981). Our sample density N+ is insufficient to determine patterns in the two major Y3 + oo eruptive cycles of the Wentworth area recognized by Gower (1988), but it is noteworthy that both terminate z with the eruption of basalts. 82 o Origin of Zr variations in the low-Zr rhyolites o o 1 We examine possible explanationsfor the observed 0 1000 2000 variation inZr content of the low-Zr rhyolites. Some of this variation may be due to differencesin Zr content of Zr the parental magma. Felsic magmas in the southeffi Maritimes Basin show geographic pb variation in iso- FIc. I l. Plot of the cation ratio (Na + K + 2Ca)/(Al.Si) against (Pe-Piper topic composition & Piper 1998) as a result Zr with regression line for rhyolites of the Fountain Lake of variation in the composition of lower crust, which Group. Also shown areregression line for the PleasantHills could also show variation inZr content.Both mafic and Pluton and field for Fisset Brook Formation.
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TABLE 2 WHOLE-ROCK CHEMICAL COMPOSIfiON OF REPRESENTATI!'E RI{YOLTTIC ROCKS OF TT{E COBEQIJID HIGHLANDS, NOVA SCOTTA
Smple 4072 4073 4074 4075 4077 4078 40E0 4109 4ll5 4116 411A 4877 6074
Major dmoB by X-ruy fluoresence (XRF)
SiO, wt% 7706 A126 7832 7642 7883 849E 7522 7807 7371 7288 7564 7644 7070 Tio, 00E 006 008 0.19 016 012 017 011 019 015 014 0.17 059 Ato, 1136 939 1106 1325 1200 880 1338 rl2l 1257 1205 ll94 1206 1240 FerOo 237 113 I t6 136 1O2 O77 152 l9E 236 213 201 154 442 I\4trO 003 OO2 002 003 002 001 002 003 004 005 003 004 008 MgO 064 085 053 085 097 087 o'75 083 241 24O 160 bd 084 CaO bd bd 0Ol 061 030 bd 010 007 002 bd bd, 016 134 NqO l4t 044 I 91 399 44r 084 3 35 266 09o 062 119 3.57 279 KrO 623 651 5 61 371 I 35 580 622 483 694 7 40 7 44 49 43O P,O, OO2 OO2 OO2 OO2 003 002 OO2 OO2 OO2 OO2 O02 OO2 023 LOI O8o 10 040 040 090 030 040 030 170 160 090 020 180
Total loooTloo6E 99.t210083 999910251101151001110086993 lOO9l 9919 9949
Tr@ elem€trtsby XRF
Ba ppm 1'14 98 143 244 44 538 347 lO7 411 473 517 16'1 609 Rb 246 26a 216 125 E3 167 162 142 155 l7l 159 174 175 Sr 23 14 33 219 150 51 144 69 38 3t 43 24 t0 Y 103 77 121 58 51 2'1 47 82 61 60 43 48 113 Zr 539 424 556 200 175 131 206 398 445 394 358 296 413 Nb 50 40 53 25 2l 19 29 47 46 46 46 37 62 Th 30 21 28 24 14 12 l'l 17 15 15 18 20 26 Pb 72 50 43 32 39 14 11 25 9 8 7 <5 23 Cn 18 22 15 16 14 13 14 21 27 25 2a 19 22 Zn 19 36 29 41 36 2'l 22 45 t6 61 54 14 145 Cu62144232512445<55 Ni4556<5714234<55 v<519561133<5<531343 Cr16477632<554<5<5<5
The rrsearth elmts md rel@tcd tre €lments by instrumentalnflfon-actiwtion u&lysis
La ppm nd nd nd nd ad nd nd nd nd nd nd nd 71 Ce nd nd nd nd nd nd nd nd n,d nd nd nd 141 Nd nd nd nd nd nd nd nd nd nd nd nd nd 73 Sn nd nd nd nd nd nd nd nd nd nd nd nd 15E Eu nd nd n.d nd nd nd nd nd nd nd nd nd 123 Ib nd nd nd nd nd nd nd nd nd- nd nd nd 32 Yb nd nd nd nd nd nd nd nd nd nd nd nd 10E Ix n.d nd nd nd nd nd nd nd, nd nd nd nd 134 Co nd nd nd nd nd nd nd nd nd nd nd nd 37 Hf nd nd nd nd nd nd nd nd nd nd nd n-d 130 Sc nd nd nd nd od nd nd nd nd nd nd nd 55 Ta nd nd nd nd nd nd nd nd nd nd nd nd 56 Th nd nd nd nd nd nd nd nd nd nd nd nd U Undtrdndndnd nd nd nd nd nd nd nd 70
Li by atomic sbsrption md F by sprcifeion €lectrodc
Li ppn nd n,d nd nd nd nd nd nd nd nd nd nd nd F nd nd nd nd nd nd nd nd nd nd nd nd 324
Formation (resultsof six analyses)show F contentstypl- Owen & J.D. Greenough,pers. commun. 1998), which cally in the range 100-700 ppm, without systematic may indicate that Cl is more abundantin this part of the geographic variation or relationship to rock type. We westem Cobequid Highlands. If so, it could account for have no Cl data for rocks of the Fountain Lake Group. the rather lower average abundancesof Zt in the West Sodic amphibolesfrom the Wentworth pluton typically Moose River road section. have a F:Cl ratio of 10-30, suggesting that F was the Petrographic observations show that the timing of dominant halogen in the eastern Cobequid Highlands. zircon precipitation influences the abundanceof Zr in Secondary(hydrothermal) biotite from the West Moose the low-Zr rhyolites. We suggest that variations in Zr River pluton has F:Cl ratio of 0.3-10 (Pe-Piper et aL abundancein the parental magma, the effect of alkalin- 1991), and scapolite veins occur near Pa.rrsboro(J.V. ity, and variable abundanceof chlorine all play a role in
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TABLE 2 (mf d) WHOLE-ROCK CHEMICAL COMPOSITION OF REPRESENTATM RHYOLITIC ROCKS OF THE COBEQIJID HIGHLANDS, NOVA SCOTIA
'1't04 Smpl€ 6083 6682 6915 7290 7291 7368 7sE9 759'I 7714 8l9l 8192 8195
I\{ajor elemots by X-ray fluoresme (XRF)
SiOrwt% 8200 7628 6698 7626 8481 7403 844t 7683 8335 7333 7998 7875 8308 Tio, 021 026 062 026 018 035 010 027 0t5 0t? o22 020 007 arq 889 119 Baz 1223 837 1272 891 1079 79't 1367 1039 939 979 FerO., 142 216 699 148 097 314 063 348 208 231 246 365 oEO MnO 004 006 0 18 003 002 008 002 007 007 005 005 007 001 Mso 083 002 I 81 004 008 009 007 002 0t2 003 028 0 19 003 CaO o52 012 054 025 011 043 bd O32 117 052 038 011 017 Nqo 019 205 250 466 038 3ll O07 262 248 314 0.96 14E 335 &o 269 632 440 334 476 5t4 497 509 225 589 541 503 343 PrO, 005 003 008 003 003 004 001 0.01 002 001 001 002 0.01 LOI 235 0'lO 234 020 090 075 r09 050 047 tt7 057 097 061
Total 99t9 999 10026 987810061998810028100 10013100291007199t610135
Trrce elmmts by XRF
Ba ppm 103 591 250 lE 96 598 lt 27 187 6E 395 <5 174 Rb 164 257 125 102 228 201 245 2t5 73 295 193 243 t7E Sr 10 34 9E 20 26 41 2t t5 62 s7 95 23 28 Y 210 67 7r 149 112 96 E7 149 130 136 110 248 13 Zr 1314 497 r8s6 1036 959 627 406 t27l 943 458 1160 1990 148 '16 Nb 130 5't 34 73 75 64 74 79 5l 90 145 33 Th 23 19 6 15 t7 16 t6 24 6 21 29 26 20 Pb 26 15 ll <10 10 t7 46 26 4t 6 83 36 127 Ga 31 t7 21 34 l8 16 19 19 17 30 lt 21 9 Zn l3l 43 142 22 23 45 t4 135 88 112 129 274 <5 Cu 5 <5 15 15't <5 <5 <5 37 12 12 5l 14 Ni 5 <5 <5 <5 <5 <5 <5 <5 <5 <5 lt <5 <5 3E <5 15 6 E <5 <5 <5 20 <5 10 20 <5 Cr 75<555<5718<5<529<56
The rresrth elmsrts ud Flected trace elmmts bv itrtmotal neukon-activation malvsis
La ppm nd nd, 86 136 nd 72 nd 126 nd 174 76 nd nd Ce nd nd 156 258 rd t43 nd 236 nd J07 155 nd nd Nd nd nd 72 115 nd 66 nd 129 nd 116 68 nd nd Sm nd nd 137 24 nd 126 nd 25 nd 22 nd ad nd Eu nd nd 26 139 nd 153 nd 114 nd 040 nd nd nd Tb nd nd 20 42 nd 22 nd 45 od 33 nd nd nd Yb nd nd 80 152 nd 96 nd 160 nd 131 nd nd nd Lu nd nd 1,30 22 nd 149 nd 20 nd 197 nd nd nd Co nd nd nd nd nd nd nd 5l nd nd nd nd n,d Hf nd nd nd nd nd nd n,d 29 nd nd nd nd nd Sc nd qd nd nd nd nd nd 061 nd nd nd nd nd Ta nd n,d nd nd nd nd nd 63 nd nd nd nd nd Th nd, nd 88 190 nd 1t0 nd 23 nd 30 nd nd nd '13 U nd nd 26 90 nd 63 nd 20 lE nd nd nd
Li by atomic abmrption md F by speific-ion elrctrode
Li ppm nd nd nd nd 6 nd trd nd nd nd n,d nd nd F 524 nd nd 510 360 192 nd nd nd nd nd nd nd
Sasplesre enlyad for 10 mqior- md minu+lment oxides ard 14 tre elmflts on a Philips PW1400 sequ$tial X-ny fluM spElrometa uing m Rh-mode X-ruy tube Major-oxid€ detmimtioG wse ffiied out on firsed gtas disks,whm qrcqrtrations of dre tr&e el@mts wqe detemined from pressed-powderp€llEts Lrtemetional stmdrds with rcomended values from Abbey (1983) as well s in-house stmdards we used for calibntion Analytiel pruision, u daemined from replicate malyses,is gmually bettq tw zyq q@fi. MgO, NEO, ud Nb, which re betts thm 5%, md Th, which is better thm 10ploLoss on ignition (L O I ) was detmined by tr€ating th€ sample for I 5 hou6 at 1050'C in m €lectric fumrce The rue-earth-elemmt mnmtrations w detmin€d by neuton-actiwtionilalysis nd : not detmined
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determining the solubility of zircon and hence the ob- is no bimodal distribution of ey6 values in the rhyolites, served variations in Zr content of the rhyolites. and intermediate differentiates with 6047Vo SiO2 are extremely rare in the study area. Petrogenesisof the high-Zr rhyolites We suggest that the observed abundancesof high- field-strength elements and the high e11acould result The high-Zr rhyolites are not simply the result of from the interaction of an intermediate or felsic differ- continued fractionation of rhyolitic magma with 40G- entiated magma with lower crust, through assimilation 700 ppmZr. Several lines of evidence suggestthat they with fractional crystallization (AFC) processes.Thls are quite distinct from the low-Zr rhyolites discussed would also account for the range of SiO2 values and the above. They show quite different trends on several el- general geochemical similarity with the older low-Zr emental plots, such as Zr versus TiO2 and cation ratio rhyolites in the more strongly lithophile elements and versus Zr. They also have high ey6 (> +3) (Fig. l2). halogens. The occurrence of the high-Zr rhyolites in compos- None of the above petrogeneticmodels can directly ite rhyolite-diabase dykes and the presenceof irregular account for the unusually high Zr conterlt of the rhyo- patchesof gabbro in the intrusive sheetsat the northem lites. The hypothesesconsidered for the low-Zr rhyo- edge of the Wentworth pluton suggest that in-mixed lites, namely variations in halogencontent of the magma mafic magma may have played a role in the evolution and variations in alkalinity (cation ratio), do not appear of the high-Zr rhyolites, particularly those with lower sufficient to produce very high Zr contents.For several SiO2 content. In the PleasantHills pluton, the granite reasons,we reject the possibility that the rhyolites were unit that shows most evidence of mingling with gabbro originally more strongly peralkaline than is suggested (unit 7) has the highest eya, +3.5, of all felsic units in by their present geochemistry and became metalu- the pluton. However, felsic rocks with Nb, Y and Zr minous as a result of devitrification. There is little scat- contents ashigh as those in the high-Zr rhyolites are not ter in Na + K, and the total alkali content is consistently found either in the Pleasant Hills pluton, nor in the lower in high-Zr dykes than in the host low-Zr rhyo- southwesternpart of the Wentworth pluton, where evi- lites. Furthermore, neither Y nor the REE arc particu- dence of mixing of mafic and felsic magmas also is larly enriched in the Cobequidht'gh-Zr rhyolites. Thus widespread(Pe-Piper et al.1996). mechanisms of the type proposed by Wilkinson er al. The high sNdvalues, together with high contents of (1996) and Mungall & Martin (1996) for peralkaline Nb, Y and Zr, may also be interpreted as derivation of a melts seem not to be applicable. felsic magma by fractional crystallization of a mafic The remaining explanation, which we consider most magma in a chamber at depth. However, unlike exten- probable, is that zircon precipitation was inhibited by sional flood-basalt settings,where such felsic fraction- high temperaturesin a metaluminous melt, as demon- atesare recognized(e.9.,Vervoort & Green 1997),there stratedby Watson & Harrison(1983, 1984).Such inhi- bition of precipitation is supportedby the petrographic observation of zircon crystallites in the fine-grained groundmass. It is also consistent with the geographic and temporal distribution of the high-Zr rhyolites in re- wa possible of heat in voluminous high- .WV lation to sources level mafic magmas.The intrusion and eruption of the 7.' .17 high-Zr rhyolites immediately precedederuption of the 16,1t e I .5-km-thick sequenceof basaltsof the Diamond Brook lo11 d2 . Formation. In the westem Cobequid Highlands, there is z 10o y no evidence for a comparableheat-source except in the q)1 Chignecto Peninsula,where mafic dykes are unusually large and abundant (Piper er al. 1993). The Chignecto 3..8 Peninsulais the only areain the westernCobequid High- lands where high-Zr rhyolite is found. -1 3o -2 CoNcLusroNs 12345 1. Felsic rocks of the Fountain Lake Group are Ga/Alx104 thickest (ca. 4 km) in the Wentworth to Byers Lake area and thin eastwardtoward Earltown (ca. 7 km) by rapid thinning of pyroclastic phasesand the disappearanceof FIc 12. Plot of eNdversrs Ga.iAl for Fountain Lake rhyolites by 1.5 km and plutonic rocks of the Wentworth and Pleasant Hills the lower eruptive cycle. They are overlain plutons. Numbers indicate sequenceof intrusion at Pleas- of continental tholeiitic basalts (Diamond Brook For- ant Hills (3 = oldest, 17 = youngest) Data from Pe-Piper & mation). In the central and westernCobequid Highlands, Piper (1998,Fig 13). the Fountain Lake Group is less than 1 km thick, and
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the formations of the Byers Lake - Earltown belt can- Br-rNcnmo, M.-C., JalrnsoN, R.A. & MoRE,E.B. (1984): Late not be recognized. Devonian - Early Carboniferous volcanism in westem Cape 2. Distinctive high-Zr rhyolites (>900 ppm Zr), a Breton Island, Nova Scotia. Can. J. Earth Sci.2I,762-774. few with accessoryamphibole, occur as a suite of dykes Buorsv, D.C. (1982): Subsidencein late Paleozoic basins rn (in places composite with diabase) cutting the entire the northern Appalachians. Tectonics 1, 1015-1037 Byers Brook Formation, as ignimbritic flows near the top of the Byers Brook Formation, as intrusive sheets CoLLNs, W.J , BenMs, S D., WHrrs, A.J.R. & Cnarperl, B.W. together with gabbros along the northern margin of the (1982): Nature and origin of A-type granites with particu- Wentworth pluton, and as a late-phasedyke in the Cape lar reference to south-easternAustralia. Contrib Mineral Chignecto pluton of the western Cobequid Highlands. Petrol. 80, 189-200. 3. The main rhyolites of the Fountain Lake Group (1999): are of similar compositions to the coeval granite plu- Dnssunrlu, G., PrpBn,D.J.W. & PE-PrIER,G. Geochemical evolution of earliestCarboniferous continen- tons of the Cobequid Highlands: they are mostly tal choleiitic basa"ltsalong a crustal-scaleshear zone, south- metaluminous, with trace-element patterns typical of western Maritimes basin, eastem Canada.Lithos (in press). A2-type granite. They show €Ndvalues similar to those of the plutonic rocks, ranging from 0 to +2. Zr contents Dorc, R., MuRpHy, J.B , Pr-Ppen, G & Pprn, D.J.W. (1996): are generally low (100-500 ppm), consistent with low U-Pb geochronology of late Paleozoic plutons, Cobequid total alkalies and possibly a high Cl:F ratio. In the West Highlands, Nova Scotia, Canada: evidence for late Moose River road section, Zr contett increases with Devonian emplacement adjacent to the Meguma-Avalon fractionation, suggesting that zircon saturation in the terrane boundary in the Canadian Appalachians Geol. J. 31,179-188. magma had not been achieved.REE patternsin samples from Squally Point were influenced by the precipitation DoNonon, H.V. & WermcB, P.I. (1982): Geological Map of of accessoryallanite. the Cobequid Highlands, Nova Scolla. Nova Scotia Depart- 4. Rhyolites of the easternCobequid Highlands typi- ment of Mines and Energy, Halifax, Nova Scotia. cally contain 300-500 ppmZr. As a result of precipita- tion of accessory zicon,Zr content decreasesslightly Dosrer-, J., Dupuv, C. & Kspprr, J.D (1983b): Uranium and in the more fractionated rocks. thorium in Paleozoic rhyolites of Nova Scotia. Can. J. 5. In the high-Zr rhyolites, precipitation of zircon Earth Sci.20,266-274 was inhibited, and zircon is restricted to small crystal- KrpprB, J.D. & Dupuv, C (1983a): Petrology and lites groundmass. in the Zircon precipitation was sup- geochemistry of Devono-Carboniferous volcanic rocks in pressed by the high ambient temperaturesof a coeval Nova Scotia. Maritime Sedimentsand Atlantic Geol. 19, mafic magma, the eruptive products of which formed 59-'71. the l.5-km-thick sequenceof basalts of the Diamond Brook Formation. DuNNrNc,G , hpen, D.J.W., Gu-Bs,P.S., Pn-Pprn, G. & Bmn, S. (1997): Chronology of the early phasesof rifting of the AcrNowr,socnveNrs Devonian-Carboniferous Magdalen Basin in Nova Scotra from U/Pb dating of rhyolites. Geol. Assoc. Can. - Min- eral. Assoc.Can, Program Abstr.22,423. This project was supported by the Canada - Nova Scotia Cooperation Agreement on Mineral Develop- DURTING,P. (1996): A seismic reflection study ofthe Fountaln ment, the Geological Survey of Canada, and NSERC Lake Group in the ScotsburnAnticline area,northem Nova grants to GPP. Craig Doucette, Mary Feetham, Jason Scotia.Geol. Sunt. Can.,Pap.96-lD,47-53. Goulden, Kathryn Parleeand Derek Robichaud assisted in the field. Constructive criticism by the Editor, Asso- Env, G.N. (1990): The A-type granitoids: a review of therr ciate Editor Dan Kontak and the referees, Brendan occurrence and chemical characteristics and speculations on their petrogenesis.Lithos 26, ll5-134 Murphy and Chris White, resulted in substantial im- provement of this manuscript. (1992): Chemical subdivision of the A-type granitoids: petrogeneticand tectonic implications. Geology ReFEnBNces 20,641-644.
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