Archean calc-alkaline lamprophyres of Wawa, Ontario ... · Precambrian Research 138 (2005) 57–87...

Precambrian Research 138 (2005) 57–87

Archean calc-alkaline lamprophyres of Wawa, Ontario, Canada:Unconventional diamondiferous volcaniclastic rocks

Nathalie Lefebvrea,1, Maya Kopylovaa,∗, Kevin Kivi b,2

a Earth and Ocean Sciences Department, University of British Columbia, 6339 Stores Road, Vancouver, Canada V6T1Z4b Kennecott Canada Exploration Inc., Canada

Received 6 February 2004; received in revised form 6 April 2005; accepted 6 April 2005

Abstract

Unusual diamondiferous rocks are found in the Wawa subprovince of the Southern Superior Craton. They are dated at2.67–2.7 Ga and comprise part of a calc-alkaline volcanic sequence of the Michipicoten Greenstone Belt. Detailed mappingof an ∼40 km2 area showed that the rocks are metamorphosed polymict volcaniclastic breccia (PVB) and lamprophyre. Thebreccia occurs as thick, 60–110 m conformable beds traceable in intermittent outcrops along strike for more than 4 km, whereasyounger lamprophyre occurs as 0.5–3 m dykes. Magmatic predecessors for the metavolcanic rocks were determined on the basisof detailed mineralogical and petrographic observations, and are found to be calc-alkaline lamprophyres. The only preservedmagmatic phenocryst phase is coarse, oscillatory-zoned amphibole of edenitic and pargasitic compositions. The parent magmasare similar in bulk composition to that of Abitibi lamprophyres and other Archean calc-alkaline lamprophyres, and may have thusalso contained phenocrystal clinopyroxene and phlogopite. The Wawa lamprophyric magmas formed contemporaneously withf mplacede eposit andc ting andp be anciente©

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elsic to mafic volcanic rocks and late orogenic intrusives of cycle 3 of the Michipicoten Greenstone Belt. They were episodically in local extensional areas, in an active Archean subduction zone. The breccia formed as a volcaniclastic dontains fragments of pyroclastic lapilli and juvenile material. Stratigraphy, a wide range in clast lithologies, poor soraucity of sedimentary structures suggests the breccia formed in a debris flow. The Wawa diamondiferous rocks mayquivalents of modern lamprophyric cinder cones and demonstrably associated epiclastic deposits.2005 Elsevier B.V. All rights reserved.

eywords:Archean; Michipicoten Greenstone Belt; Diamonds; Calc-alkaline lamprophyre; Volcaniclastic; Debris flow

∗ Corresponding author. Fax: +1 604 822 6088.E-mail address:[email protected] (M. Kopylova).

1 Present address: De Beers Canada Exploration, 1 William Mor-an Drive, Toronto, Ont., Canada M4H 1N6.2 Present address: Kivi Geoscience, 309 S Court St., Thunder Bay,nt., Canada P7B 2Y1.

1. Introduction

Primary economic diamond deposits have bfound in only two volcanic rock types, kimberlitand lamproites. They occur in Archean cratonsProterozoic mobile belts (Helmstaedt and Gurne1995) as late, cross-cutting units with Proteroz

301-9268/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.precamres.2005.04.005

58 N. Lefebvre et al. / Precambrian Research 138 (2005) 57–87

to Cenozoic ages (Kirkley et al., 1991; Heaman etal., 2004). Recently, an unusual suite of diamond-bearing metavolcanic rocks was discovered in theWawa and Abitibi subprovinces of the Superior Cra-ton (Ayer et al., 2003; Vallancourt et al., 2003; Fig. 1).The rocks of the Michipicoten Greenstone Belt inWawa are the target of focussed diamond explo-ration by major and junior mining companies. Todate, thousands of stones have been recovered, 95%of which are microdiamonds (<0.5 mm in one dimen-sion) with grade estimates ranging from 0.2 to 1 ct/t(Buckle, 2002). The macrodiamonds display a vari-ety of colours, a predominantly octahedral morphol-ogy (Lefebvre et al., 2003), and very little mechan-ical wear, all typical of diamonds hosted in primaryvolcanic rocks.

This recent diamondiferous finding has severalunique characteristics with scientific and economic sig-nificance. Firstly, their emplacement age is Archean(Stott et al., 2002; Ayer et al., 2003), thus makingthem one of the oldest known primary diamondif-erous rocks. Secondly, some of the diamondiferousrocks are reported to be shoshonitic lamprophyres(Ayer et al., 2003; Vallancourt et al., 2003), i.e. theK-rich variety of calc-alkaline lamprophyres (Rock,1991), thus making them the first confirmed occur-rence of diamonds in calc-alkaline rocks. Finally,the subduction-zone setting of the Wawa and Abitibidiamond-bearing volcanic rocks (Sage, 1994and ref-erences therein), while not unknown (Barron et al.,1 0i aryd

andp nicr ortho mi-n rth

and northwest of the mapped area (inset ofFig. 1).We identify two types of diamondiferous metavolcanicrocks—breccias and intrusive dykes—each with dis-tinct styles of emplacement. We provide petrographicconfirmation that the diamondiferous rocks are calc-alkaline lamprophyres, a new unconventional diamondsource. We also examine the volcanology of the brec-cias and find modern analogues for the tectonic settingand volcanic processes that created the Wawa diamon-diferous rocks.

2. Regional geologic and tectonic setting

The Michipicoten Greenstone Belt is considered awestern extension of the Southern Volcanic Zone of theAbitibi greenstone belt (Ludden et al., 1986), but differsfrom it in that the mafic and felsic volcanic rocks recordthe three cycles of igneous activity (approximately2.89, 2.75 and 2.70 Ga;Turek et al., 1982, 1992), versusone (approximately 2.75–2.7 Ga) in the latter. All of theMichipicoten cycles are bimodal basalt–rhyolite suites;the 2.89 Ga volcanic units also contain komatiites (Sageet al., 1996). The third cycle of volcanism is representedby massive and pillowed, intermediate to mafic, tholei-itic lava flows, conformably overlain by intermediateto felsic tuff, breccia and clastic sedimentary rocks(Williams et al., 1991; Sage, 1994). Intrusive rocks gen-erated by this cycle of magmatism include gabbro toquartz–diorite sills and dykes (Sage, 1994) and syen-i eB f theKw andt ler ill,1 ingo

F n Green nces of theS of the sW ons of c rovinces( viously diamondo 3 (mo res k. Sour 58 and1 t al., 20 ; (4)c blished ;A

996; Capdeliva et al., 1999; Griffin et al., 200),s nonetheless an extremely rare setting for primeposits.

We undertook detailed study of the geologyetrology of the Wawa diamondiferous metavolcaocks on the Band-Ore Resources property, 20 km nf Wawa, (Fig. 1), along with reconnaissance exaation of outcrops of similar rock types to the no

ig. 1. Location of the Wawa subprovince within the Michipicoteuperior Craton (Card and Ciesielski, 1986). A star shows locationilliams, 2003) within the Abitibi subprovince and dots—locati

Wyman and Kerrich, 1989). The inset illustrates locations of preccurrences (circles) and volcanic and intrusive rocks of cyclehown in grey, PVB and lamprophyres of map area (Fig. 2) are blac45 (Stott et al., 2002); (2) breccia of Cristal occurrence (Stachel ealc-alkaline dacite (Ayer et al., 2003); (5) felsic tuff (Ayers, unpuyer et al., 2003); and (7) lamprophyre dyke (Ayer et al., 2003).

tes (Stott et al., 2002). The Michipicoten Greenstonelt was strongly deformed by the Wawan phase oenoran orogeny (approximately 2.67 Ga;Stott, 1997),hich resulted in large-scale recumbent folding

hrusting, followed by upright folding and high-angeverse faulting (Arias and Helmstaedt, 1990; McG992). This deformation has produced local stackf stratigraphy (Williams et al., 1991). A four-stage

stone Belt in the Superior Craton. Dashed lines divide subprovihoshonitic diamondiferous lamprophyres (Wyman and Kerrich, 1993;alc-alkaline lamprophyres within the Uchi and Wabigoon subpdated samples of PVB (squares), lamprophyres (diamonds),

dified afterVallancourt et al., 2003). Felsic volcanic rocks of cycle 3 aces for the U–Pb ages are: (1) Lamprophyre dykes at outcrops04); (3) breccia of Mumm occurrence (Ayers, unpublished data)data); (6) lamprophyre of the Sandor occurrence (Stott et al., 2002

N. Lefebvre et al. / Precambrian Research 138 (2005) 57–87 59


Table 1Lithological and mineralogical data for the polymict volcaniclastic breccia, lamprophyre, and juvenile material

Matrix-supported breccia Clast-supported breccia Lamprophyre Juvenile material

Location (Fig. 2) E-1, E-2, SE-F, DE-1,DE-2, DE-3, JR-14,BR-1, BZ, 3648 and 51

E-2, BZ, 52 145, 58, 52, 51, 3647,3649, BZ

DE-1, 51, 52, BZ

Clast abundance Commonly ranges from10 to 15% but can be aslow as 0 to 10%

≥45% Commonly ranges from 5to 10% but can be as highas 15% and as low as 1%.

Absent

Bedding Present at E-2, 51, DE-1(medium to thickbedding)

Present at 51, BZ Absent Absent

Crude grading Present at E-2, DE-1, BZ Absent Absent AbsentMineralogya Breccia matrix: 50–75%

(1–10% >0.1 mm) Act,1–20% Ep, 1–20% Ttn,0–20%, Bt, 1–15% Ab,0.5–15% Hbl, 0–10%Chl, 1–3% Cal, trace(<1%): Ap, Ms, Qtz, Chr,Rt, Zrn, Py, Ccp, Lm, Po,Fe oxide, oligoclase andleucoxene

Breccia matrix: 50–75%(1–10% >0.1 mm) Act,1–20% Ep, 1–20% Ttn,0–20% Bt, 1–15% Ab,0.5–15% Hbl, 0–10%Chl, 1–3% Cal, trace(<1%): Ap, Ms, Qtz, Chr,Rt, Zrn, Py, Ccp, Lm, Po,Fe oxide, oligoclase andleucoxene

50–80% Act, 1–15% Ep,1–10% Ttn, 1–30% Bt,1–20% Ab, 0–7% Hbl,0–5% Chl, 0–10% Cal,0–5% Mc, 0–5% Qtz,trace (<1%): Zrn, Ap, Ms,Chr, Ccp, Py, Rt and Feoxide

45–80% Act, 5–20% Hbl,5–30% Ttn, 1–20% Ep,0–10% Ab, 0–3% Cal,2–3% Chl, trace (<1%):Bt, Ap, Zrn, Py, Ms, Rt,Chr and oligoclase

a Mineral abbreviations in tables are fromKretz (1983).

deformational history has been previously recognizedby Arias (1996). In the study area, evidence for twoof these deformation events is preserved as S2 and S4foliations.

Syn- and post-Kenoran magmatism in the Michipi-coten is represented by four events: lamprophyre dykeintrusions at 2.7–2.67 Ga (Stott et al., 2002), graniteintrusions at 2629–2650 Ma (Percival and West, 1994),the Matachewan diabase dyke swarm at 2454 Ma(Osmani, 2001) and the Keewenawan dyke swarm at1142 Ma (Vallancourt et al., 2003). The first event,which is the focus of this study, is of widespread dis-tribution. Lamprophyre dyke emplacement occurred inthe Abitibi Greenstone Belt at 2687–2675 Ma (Wymanand Kerrich, 1993; Wyman and Kerrich, 2002; Ayer etal., 2003), and over a wider range within the SuperiorCraton between 2.7 and 2.67 Ga (Barrie, 1990; Sternand Hanson, 1992).

At present, two tectonic models have been set forthas possible histories for the Early to Middle Archeanrocks of the Superior province. The first suggests theSuperior province may have formed by repeated accre-tion of terranes as a result of subduction in a compres-sional margin (Hoffman, 1989; Williams et al., 1991).

This model is supported by seismic, structural and geo-logical data (Calvert et al., 1995; Calvert and Ludden,1999; Thurston, 2002). Under this model, deformationwithin the Michipicoten Greenstone Belt resulted fromsubsequent accretion of volcanic arcs during formationof the belt, and by accretion of the Wawa subprovinceto the Superior Craton nucleus (Arias, 1996). The vol-canic rocks of Wawa are interpreted to be allochthonousassemblages of island and continental arcs (Sylvesteret al., 1987), tectonically transported to their presentposition (Thurston, 2002). The competing model advo-cates an autochthonous origin for the MichipicotenGreenstone Belt, with greenstones being accumulatedin place, erupting through and being deposited uponolder units (Thurston, 2002; Ayer et al., 2003). Underthis model, the Superior Province would have experi-enced orderly, autochthonous progression from plat-forms through rifting of continental fragments, andlate assembly during the Kenoran orogeny. This inter-pretation of all cycles of Michipicoten volcanics asintra-cratonic magmatism is supported by geochemi-cal evidence, which records crustal geochemical sig-natures and significant contributions from continentalpassive margin sources (Sage et al., 1996).


Fig. 2. Geological map of the study area, with sampling and drill hole locations. UTM coordinates are in NAD27.


Fig. 3. Detailed (1:100 scale) trench map BR-1, showing lithology, size, angularity and distribution of the fragment population. Fragmentlithology: (1) intermediate to mafic instrusive rock; (2) unknown; (3) felsic metavolcanic rock; (4) altered hornblende-rich rock; (5) alteredgreenstone; and (6) mafic metavolcanic rock.

3. Geology of the mapped area

The study area was mapped at 1:10,000 scale(Fig. 2). Also, 7 outcrops and nine trenches of diamon-diferous rocks were mapped using a modified versionof the Einaudi (2000)Anaconda mapping method ata scale of 1:100 (Lefebvre, 2004). An example of oneof these detailed maps is provided inFig. 3. The col-lected data on structure, lithology, and clast populationand distribution within the breccia are summarized inTables 1 and 2.

The area is underlain by cycle 3 mafic to fel-sic metavolcanics, previously dated at 2699–2701 Ma(Fig. 1 inset), with subordinate interflow and vol-caniclastic sediments and thicker beds of metasedi-ments. The entire package is overturned, as evidencedby sedimentary structures in the southwestern por-tion of the study area, by the distribution and size ofamygdules, and by orientation of pillow structures inmafic metavolcanic rocks. To the northeast, the pack-age is juxtaposed against mafic metavolcanics alongan unnamed thrust fault. Further to the northeast, the


Fig. 4. Simplified geological cross-sections based on subsurface drilling (this study). All sections face north–west. Section C is a compilationof results from drill holes intersecting a single stratigraphic marker (electromagnetic horizon) at several locations along its strike. Note thedistribution of larger amygdules and pillow structures in metavolcanic rocks, indicative of the overturned volcanic sequence.

Mildred Lake fault separates two packages of maficmetavolcanic rocks, which are inferred to be tectonicstacking of a single unit on the basis of recurrence of anelectromagnetic marker horizon (Fig. 2). The markerhorizon, which consists of graphite-rich argillite sedi-ments, was mapped geophysically, geologically and bydrilling (Fig. 4).

Two main types of diamondiferous metavolcanicrocks are recognised, based on lithological and min-eralogical data presented below. These are metamor-phosed polymict volcaniclastic breccia and lampro-phyre (Table 1).

Discontinuous exposures of the polymict volcani-clastic breccia have been discovered over at least50 km2 within the Musquash, Menzies, Lalibert andLeclaire townships of north-western Ontario (Vaillan-court et al., 2003). The largest outcrop of exposedbreccia is 1500 m× 500 m (Walker, 2003). In the studyarea, the breccia occurs as thick beds (maximum truethickness∼110 m) which dip to the NE at 30◦, andwhich have conformable upper and lower contacts withmetavolcanic rocks (Figs. 4 and 5A). The breccia isalso found as infill of fractures in large boulders ofcountry rock, along contacts with mafic metavolcanics,


Fig. 5. Field photographs of polymict volcaniclastic breccia. (A) Irregular, sharp contact of matrix-supported polymict volcaniclastic breccia(PVB) with intermediate to mafic metavolcanic rock (MV) which extends laterally beyond field of view. Note within the matrix-supportedbreccia, an irregular shaped angular boulder-sized fragment “F” of clast-supported breccia (outcrop 51). (B) Fragment F of clast-supportedbreccia (enlarged from A) is dominated by granule to pebble-sized, angular intermediate to mafic metavolcanic fragments. Outlined are severalrounded cored pyroclastic lapilli (outcrop 51). (C) Thin, pebble- to cobble-sized, tabular and oblate shaped bodies of matrix-supported PVBpenetrating mafic intrusive rock (MI) close to their contact (outcrop 3647). (D) Fragments of matrix-supported breccia PVB-MS, which containless than 5% metavolcanic and/or intermediate to mafic intrusive fragments (outlined by a dashed line) within the clast-supported breccia PVB-CS(outcrop 52). (E) Hornblende-rich mantle xenolith MX (white dashed line) is coated by matrix-supported breccia material PVB-MS (outlined bya line of shorter black dashes) within clast-supported breccia PVB-CS (outcrop 52). (F) Cobble-sized, irregularly shaped fragment of juvenilemagmatic material (JM) within PVB (trench B–Z). (G) Pebble-sized intermediate-to-mafic intrusive fragment exhibiting jigsaw-fit texture, i.e.fractured clasts that are slightly scattered but the pieces can still be fitted back together like a jig-saw puzzle. (H) Tabular shaped beds (dashedoutlines) within the volcaniclastic breccia marked by sharp contacts and variation in clast abundance. (I) A bed of PVB exhibiting crude grading,which is marked by a concentration boulder-sized fragments (dashed outline) at the upper bedding contact. Grading in the other size fractionsof material is not evident. (J) Block of “soft-sediment deformation” structure (∼1 m in size) occurs as folded beds marked by variation in clastcontent within the clast-supported PVB (trench B–Z).


Fig. 5. (Continued).

and intricately penetrating intermediate to mafic intru-sive rocks close to the breccia contact (Fig. 5C). Bedsof breccia occur at several stratigraphic levels, a rela-tion, which is best observed where they are foundboth above and below the electromagnetic marker hori-zon in a continuous cross-section (Fig. 4C). The bedintersected by drill holes 03GQ-09 and 03GQ-08 isdated at 2724± 24 Ma (Stachel et al., 2004), but otherbreccia beds west of the drill holes may be younger.These breccia beds occupy a similar stratigraphic posi-tion to that of the 2687–2680± 1 Ma breccias (Fig. 1inset) to the northwest of the study area. The young-ing of breccias in the western part of the study areais supported by observed field relationships, and bytheir higher stratigraphic position in this overturnedvolcanic–sedimentary sequence. The breccia in the NE

of the map area is coeval with some mafic flows, butmany other breccia beds are younger than the metavol-canic rocks, as evidenced by the inclusion of their frag-ments in PVB (Table 2). Intermediate to mafic intrusiverocks post-date the breccias in the NE of the map area,where megaliths of breccia are found within a gabbrointrusive (Fig. 4B). In all other areas to the SW, brecciascontain fragments of intermediate to mafic intrusives.

Lamprophyre is present in the SW, SE and NW ofthe property (sample locations 58, 145, 52, 51, 3547,3649 onFig. 2), where it occurs as narrow dyke-likebodies (Fig. 6A) or in bodies of indeterminate mor-phology (Fig. 6B), which cross-cut or are intercalatedwith country rock. The lamprophyre in the study areahas been dated at 2674± 8 Ma, and age determinationsbetween 2715 and 2679 Ma are reported for lampro-


phyres in the vicinity (Fig. 1 inset). The lamprophyredykes are in observable cross-cutting relationship toeach of intermediate-to-mafic intrusive rocks, felsicmetavolcanic rocks, and PVB (Fig. 6C). However,in one area (outcrop 52,Fig. 2), contemporeneity oflamprophyre with an intermediate-to-mafic intrusiveis inferred by the presence of a hybrid contact rock(Fig. 6D), composed of bleached prolate lenses of thewallrock intermixed with lenses of lamprophyric mate-rial. Petrographic examination of samples from thiscontact exhibit mixing on a microscopic scale, withlenses of intermediate material enriched in quartz andalbite (Fig. 8D). This contact appears to be a quenchedstructure which preserves mingling of the two magmas(e.g.Paim et al., 2002). Lamprophyre with indetermi-nate morphology is found to predate intermediate-to-mafic intrusive rocks, felsic metavolcanic rocks, andPVB, as fragments of each rock types are found withinit (Table 2; Figs. 3and 6E). However, fragments of lam-prophyre have been observed within the intermediate-to-mafic intrusive rocks near the contact (Fig. 6F),implying multiple episodes of mafic magmatism.

The map area also includes intrusions ofintermediate-to-felsic intrusives (gabbro to quartzdiorite), which are cut by northwest- and northeast-striking diabase dykes. These younger intrusiverocks likely belong to the Matachewan, Hearst andKeewenawan swarms (Vallancourt et al., 2003).

4

inlyo ofi nedm eda y oft oul-

ders (up to 9 m), and fragments of similar size butdifferent density occur within the same area (Fig. 3). Itcontains rare primary sedimentary structures, includ-ing bedding, crude grading and structures of folding insemi-consolidated material (Table 1). Irregular-shapedpods or lenses, marked by subtle to obvious changesin clast abundance, are common. Bedding planes areidentified by variations in clast abundance or in relativeabundance of felsic metavolcanic fragments (Fig. 5H).Beds are 0.3–1 m thick, tabular to wedge shaped, havesubtle or sharp contacts, and extend laterally up to∼10 m. The majority of the beds are massive; rare bedsexhibit subtle grading, marked by a concentration ofsmall boulder-sized fragments along bottom beddingplanes (Fig. 5I). The distribution and size of amyg-dules in nearby mafic metavolcanic rocks indicate thatthese beds are overturned, and therefore, the observedgrading is inferred to be normal (Fig. 4). Soft sedimentdeformation structures up to 1 m in size occur as foldsof clear, fine-grained, laminated and bedded horizons,which exhibit grading from sand to pebble-sized mate-rial (Fig. 5J).

At least eleven different lithologic fragment typesare irregularly distributed throughout the breccia(Table 2) and are commonly aligned in the plane of foli-ation. The clast population is dominated by lithologieswhich are locally derived, and with which the breccia iscommonly intercalated (i.e. mafic and felsic metavol-canic rocks, and intermediate to mafic intrusive rocks).Other clast types include fragments of clast-supportedb sts?F ciaw sc rial( afici ar-g ri en

F rophyr Sharp,s amprop black andw cia (PV by a blacka fic intru ated byt h brecc ody withi lampro mediate-t inate m rite-richx . Mono of darkerr yre dy

. Polymict volcaniclastic breccia

The polymict volcaniclastic breccia consists maf angular, pebble-sized fragments, dominantly

gneous origin, all set in a greenish-grey, fine-graiatrix (Fig. 3). The breccia is massive, unstratifind poorly sorted with respect to size and densit

he clasts. Clast size ranges from sand to large b

ig. 6. Lamprophyre field photos. (A) Thin off-shoots of lamptraight contact between matrix-supported breccia (PVB) and lhite line. (C) Irregular contact between matrix-supported brecnd white line. (D) Lamprophyre intruding intermediate to ma

he presence of contact hybrid rock. (E) Fragment of clast-ricndeterminate morphology (L). (F) Angular, triangular shapedo-mafic intrusive rock (MI). (G) Lamprophyre body of indetermenoliths, which are less weathered than the lamprophyre. (H)ock enriched in biotite within the lamprophyre dyke. Lamproph

reccia within matrix-supported breccia (autoclaig. 5A), fragments of older matrix-supported brecith <5% fragments (Fig. 5D), and lithic fragmentoated with similar matrix-supported breccia mateFig. 5E). Many supracrustal and intermediate-to-mntrusive fragments are blocky, have curviplanar mins and jigsaw-fit texture (Fig. 5G). Of particula

mportance is the presence of material that has be

e dyke (L) cross-cutting felsic metavolcanic rock (FMV). (B)hyre with indeterminate morphology (L). Contact outlined by aB) and cross-cutting lamprophyre dyke (L). Contact outlinedsive (IMI) rock. The contact is irregular and strikingly accentuia (F), outline by a black and white line, within lamprophyre bphyre lense (L), outlined by a black and white line, within interorphology with round, monominerallic actinolite (Act) and chlominerallic actinolite xenoliths (Act) enveloped by a distinct rimke is cross-cutting intermediate to mafic intrusive rock (MI).

68N.Lefebvre

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138(2005)57–87

Table 2Abundance, shape, angularity, size, and texture of clasts present in the polymict volcaniclastic breccia and lamprophyreClast typea Matrix-supported breccia Clast-supported breccia Lamprophyre

Characteristicsb 1 2 3 4 5 Comments 1 2 3 4 5 Comments 1 2 3 4 5 CommentsFelsic metavolcanic ++ I, B, O,

P, EA-SR SD-C F May exhibit

jig-saw-fittexture,jointing,magmatic rinds

++ I, P, O A-SR G-C F + B, O, I A-SR G-B F May contain coarse Pycrystals

Mafic metavolcanic ++ O, I, E,P

A-SR SD-C F May containcoarse pyritegrains, joints,magmatic rinds

++ O A-SR G-C F May containmagmatic rinds

+ O SA-SR P-C F

Intermediate tomafic intrusive

++ I, B, O,P, E

A-R G-B M-C May exhibitjigsaw-fittexture,magmatic rinds

++ O, I, E,B, P

A-SR G-B M-C + O, I, E A-R P M-C

Biotite-richgreenstone

++ O, B, E A-SR G-C F-M May containcoarse pyritegrains

+ O, E, P,B

A-SR P-C F-M May containmagmatic rinds

++ B, O A-SR SD-B F May have rim of Bi-richlamprophyre

Greenstone ++ I, O, B,E

A-SR G-B F May have rindsof clast-poorbreccia material

+ O SA-SR P-C F May have rim of Bi-richlamprophyre

Hornblende-rich + O, E,B, P

A-R G-C M-C Mantle xenolith(high Cr and Nicontent)

+ E, B, O A-SR P M-C May containmagmatic rinds

++ E, B R-SA P-C C Mantle xenolith (high Ni andCr content); rare xenolithshave Act cores

Juvenile magmaticmaterial

+ O, I SR-R G-C F + I R-SR G-C F

Gneiss + O SA-R P-B M-C + B A C-B M-CActinolite-rich + O R-A C-B M-C ++ O, E, B R-A P-C C Radiating Act grains; may

have rim of Bt-richlamprophyre

Greenschist + O, P, E A-R P-C F + E, P, O A-SR P-C F May containmagmatic rinds

+ P, O A-SR P-C F

Clast-supportedbreccia

+ I A-SA P-C F

Matrix-supportedbreccia

+ O SA-SR P-C F + O, E,B, I

A-SR P-B F

a The type of clast was determined by petrographic and field observations based on remnants of igneous textures. Where recognition of protolith was not possible, the clasts arecharacterized according to their metamorphic mineral assemblage.

b 1—Abundance: ++, common to dominant; +, common to sparse. 2—Shape (based on nomenclature byZingg, 1955): O, oblate; P, prolate; E, equant; B, bladed; I, irregular.3—Angularity (Powers, 1953): A, angular; SA, subangular; SR, subrounded; R, rounded. 4—Size (based on size nomenclature byWentworth, 1922): B, boulder; C, cobble; P,pebble; G, granule; SD, sand. 5—Texture (based on nomenclature byWinter, 2001): F, fine-grained; M, medium-grained; and C, coarse-grained.


interpreted as juvenile magmatic fragments, occur-ring as irregular, round, and ovoid aphanitic fragments(Fig. 5F), or as magmatic “rinds” up to 3 cm thick onaccessory lithic fragments (Fig. 5B). These could berare examples of primary pyroclastic textures. Juve-nile fragments and rinds can be distinguished fromolder breccia fragments and coatings by the absenceof metavolcanic and/or intermediate-to-mafic intrusivefragments.

The matrix/clast ratio is highly variable within thebreccia (Table 1), and the breccia occurs both as matrix-supported and clast-supported types. The matrix grain-size ranges from <2 mm to 1�m. PVB is cut by veins ofchlorite, epidote, iron-oxide, kaolinite, quartz, and cal-cite. Two foliations were observed (Lefebvre, 2004): adominant, shallow to moderately NNE dipping folia-tion S2; and a weaker, shallow to steeply ESE dippingfoliation S4, which crenulates S2 (correlated with workof Arias, 1996).

5. Lamprophyre

Lamprophyre dyke width ranges from approxi-mately 50 centimetres to 3 m, with local offshoots of2–3 cm thickness (Fig. 6A). Dyke contacts vary fromsharp and straight (Fig. 6B and H) to highly irregular(Fig. 6A). Variations in grain size and colour, whichwould indicate high-temperature alteration (chill mar-gins), were not observed at dyke margins. The lam-p rallyc ble-s int i-na onea entsm fdp ticb werl eo nceo cti-n hsn jor-i ricd

Lamprophyres also occur in bodies of indeterminatemorphology, which could be either dykes or lava flows.Additional structural studies are required to establishtheir geology. The lamprophyre bodies with indeter-minate morphology are oriented parallel to the planeof foliation observed in the polymict breccias withwhich they are in contact (Fig. 6B). Lamprophyres ofindeterminate morphology differ in characteristics ofxenolithic fragments from lamprophyre dykes. Firstly,the indeterminate bodies do not have biotite-rich haloesaround xenoliths (Fig. 6G). Secondly, these xenolithsshow positive relief on outcrops not seen in dykes (com-pareFig. 6G and H). Lastly, fragment lithology is morediverse in indeterminate morphology bodies than indykes where only three types of xenoliths are found.

6. Petrography

6.1. Polymict volcaniclastic breccia

The polymict volcaniclastic breccia exhibits afragmental texture in thin section (Fig. 7A). Opticalmicroscopy confirms that there are 11 types offragments, as identified in the field (Table 2). Thefragments are contained within an inequigranularmatrix, consisting of actinolite, chlorite, albite,±titanite, ±epidote,±biotite (Table 1). This assem-blage is typical of greenschist facies mafic rocks(Yardley, 1995). In the matrix, coarser (0.2–1.5 mm)grains of hornblende, biotite and epidote are set ina nga Sf ed,a tet e,e pre-d do ping( ,s entc ingf l toe ndedb ino-lT owo ssr

rophyre is fine-grained, grey–black, and geneontains 5–10% subrounded to subangular, cobized fragments. Eight lithologies were identifiedhe fragments (Table 2). The clast population is domated by actinolite-rich, monominerallic rocks (Fig. 6Gnd H), or less commonly by biotite-rich greenstnd hornblende-rich ultramafic rocks. Some fragmay be enveloped by a distinct rim (∼1–5 cm thick) oarker lamprophyre, enriched in biotite (Fig. 6H). Lam-rophyre is distinguished from polymict volcaniclasreccia by: (1) lower content of fragments and fe

ithological clast types (Tables 1 and 2); (2) the absencf the juvenile magmatic material; (3) a predominaf highly altered, coarse-grained, monominerallic aolite fragments; (4) paucity of wall-rock xenolitear contacts; (5) rounded morphology of the ma

ty of xenoliths; and (6) a diminished degree of fabevelopment, including absence of S2 foliation.

finer-grained (≤0.1 mm) groundmass containill of the above minerals except hornblende. The2

oliation is defined by the alignment of fine-graincicular actinolite± chlorite grains, which domina

he groundmass (Table 1). Coarser-grained biotitpidote, and subhedral to euhedral hornblendeate or grew synchronous with S2 deformation, basen chlorite pressure shadows and texture wrapFig. 7B and C; Passchier and Trouw, 1996). Rareubhedral, tabular biotite grains commonly show bleavage with fine-grained acicular actinolite growrom the grains into the fabric. Coarse subhedrauhedral hornblende grains are commonly surrouy coronas of, or are partly replaced by, act

ite ± biotite± chlorite± titanite± albite± calcite.he fresher hornblende grains commonly shscillatory (Fig. 7D) or patchy zonation. Groundmautile is commonly mantled by titanite coronas.


Fig. 7. Photomicrographs of the PVB. (A) Fragmental texture of the volcaniclastic breccia. Three fragment types are shown by lines with differentpatterns and noted as J (juvenile magmatic material), M (intermediate to mafic intrusive rock), and F (intermediate to felsic metavolcanic rock).Note that the fragments are preferentially aligned parallel to foliation. The breccia matrix is inequigranular and comprises larger grains ofhornblende, biotite and epidote within the fine-grained groundmass. (B) Coarser grained epidote wrapped by the S2 fabric. (C) Coarser grainedoscillatory-zoned hornblende with chlorite pressure shadows and the S2 fabric wrapping around the grain. (D) SEM photo of coarser grainedoscillatory-zoned hornblende.


Mineralogy of the breccia matrix varies; actinolite-dominated matrix is most representative, but chlorite(trench E-1;Fig. 2) and biotite (trenches BZ, JR-14,DE-3, E-2, E-1;Fig. 2) locally form the dominantphase. In trench E-1, chlorite-rich breccia appears tobe associated with 1–3 cm thick quartz veins, which

cross-cut both the breccia and the surrounding inter-mediate to mafic metavolcanic and/or intrusive rocks.The breccia matrix and the fragments within the brec-cia closest to these veins are dominated by calcite,albite and chlorite, the latter defining the S2 foliation.Two meters away from the quartz veins, the breccia

Frbaci

ig. 8. Photomicrographs of the juvenile magmatic material and lamprock (M) enveloped by a rind of juvenile magmatic material (J) whichoundaries between the three rock types. (B) SEM photo of the finer-gctinolite, chlorite and albite. Hornblende is mantled by actinolite and chomprises biotite, albite, actinolite, and chlorite. Accessory zircon, e

ntermediate intrusive (F) and lamprophyric (L) material in outcrop 51

ophyre. (A) An accessory lithic fragment of intermediate to mafic intrusiveis contained within the breccia matrix (BM). Note the sharp, distinct

rained matrix (≤0.1 mm) of the juvenile magmatic material, dominated bylorite. (C) Fine-grained (≤0.1 mm) matrix of the lamprophyre dominantly

pidote, apatite, titanite and quartz are also present. (D) Interminglingof.


matrix becomes richer in biotite, which controls the S2foliation and forms pseudomorphs after coarser horn-blende grains, together with chlorite and actinolite.A diffuse contact between the biotite-dominated andthe actinolite-dominated breccia matrix was identifiedseveral meters from the quartz vein through detailedmapping.

6.2. Juvenile magmatic material

Juvenile magmatic material occurs as discrete elon-gate and ovoid fragments (2–6 mm;Fig. 7A) and rims(3–20 mm) on other clasts in the PVB. The rims haveobvious, sharp contacts with the breccia matrix andwith the lithic fragments they enclose (Fig. 8A). Thejuvenile magmatic material is comparable mineralog-ically and texturally to the breccia matrix, comprisingactinolite, titanite, chlorite and albite (Fig. 8B). Thematerial differs from the breccia matrix in that it con-tains: (1) coarser and more abundant actinolite; (2) lessof the larger epidote (0.2–1.5 mm) and fine-grained pla-gioclase grains, and more fine-grained oligoclase andmuscovite (<5 vol.%); and (3) more coarser grainedoscillatory-zoned hornblende.

6.3. Lamprophyre

The inequigranular texture of the lamprophyreis defined by larger (0.1–4 mm) grains of horn-b seti -mt assa ndh ostc oft omor-p -b veryr thel on-t por-t m).B ro-p d-m eenm

7. Analytical methods

We present major element geochemistry and micro-probe mineralogy data for over 100 samples collectedduring field mapping, which form the basis of our pet-rographic and mineralogical discussion. Minerals wereanalyzed using an automated CAMECA SX-50 wave-length dispersive electron microprobe (Departmentof Earth and Ocean Sciences, University of BritishColumbia, Canada) and were treated with the ‘PAP’�(�Z) on-line correction program. Silicates and oxideswere analyzed at an accelerating voltage of 15 mV anda 20�A beam current, except for a few samples offine-grained plagioclase which were analyzed at a beamcurrent of 10�A. On-peak counting times for most ele-ments in mica, amphibole, chromite and epidote were20 s, K in mica and amphiboles was counted for 80 s,and V in amphiboles for 40 s. On-peak counting timesfor most elements in plagioclase was 10 s except for Fe,which was counted for 30 s. Analyses with poor stoi-chiometry and totals were excluded. Mineral compo-sitions were averaged for homogeneous phases, or arepresented as individual analyses for inhomogeneousminerals (Tables 3–5). Ferric iron in amphibole wascalculated using the methods ofSchumacher (1997).

The whole-rock geochemical analyses were doneat the McGill University Geochemical Laboratories(Montreal, Canada). The samples (80–120 g) wereground in a jaw crusher and then in a tungsten-carbidering mill with minimum grinding times. All major ele-m inedb g aP ilicaw s.%,a andt l pre-c l.%.

8

eso olite( fora( mi-n e-r nde

lende, actinolite, epidote and biotite (5–10%)n a finer-grained (≤0.1 mm), hypidioblastic ground

ass (Table 1 and Fig. 8C). The weak S4 folia-ion is controlled by the alignment of groundmctinolite, biotite,± chlorite. Subhedral, tabular aexagonal biotite with bent cleavage is the mommon porphyroclastic mineral. The majorityhe coarser-grained hornblende has been pseudhed by biotite± actinolite± chlorite. Where hornlende grains are preserved, oscillatory zoning isare to absent. In contrast to the breccia matrix,amprophyre lacks oscillatory-zoned hornblende, cains groundmass microcline, and a higher proion (10–15%) of larger biotite grains (0.2–1.3 miotite-rich rims that surround xenoliths in the lamphyre (Fig. 6K) have diffuse contacts with the grounass and are interpreted as reaction rims betwetastable xenoliths and the magma.

ents, Cr, Co Ba and Ni contents were determy X-ray fluorescence (XRF) spectrometry usinhilips PW2400 spectrometer on fused pellets. Sas determined with standard accuracy of 0.5 abll other major elements with 1 abs.% accuracy,

race elements with 5 abs.% accuracy. The generaision of the XRF spectrometry is better than 0.5 re

. Mineral compositions

All of the rock types analyzed contain two typf amphibole: hornblende and secondary actinTable 3). The hornblende compositions, similarll three rock types, are characterized by low Cr2O3<0.15 wt.%), with edenite and pargasite as the doant types (Fig. 9). Hornblende in the juvenile matial is less commonly pargasitic. Edenitic hornble

N.Lefebvre

etal./P

recambrianResearch

138(2005)57–87

Table 3Representative electron microprobe analyses of amphibole compositionsRock type Polymict volcaniclastic breccia

Grain typea MM MO P MA MC MR CB OC OIR, OOR OIR CB OOR MM CB OC OIR OOR MC, MR MM

Number of analyses 7 6 3 7 6 3 2 13 23 1 1 3 1 2 9 19 2 5 1

SiO2 54.43 53.68 54.28 54.53 44.71 43.49 44.25 44.88 44.50 42.29 46.31 45.30 51.15 42.75 42.90 42.81 42.83 42.16 43.35

TiO2 0.03 0.05 0.04 – 1.14 1.11 1.60 1.32 1.16 3.24 1.60 0.86 – 1.18 1.33 1.32 1.64 1.67 1.43

Al2O3 1.44 2.06 1.46 1.44 9.60 9.83 9.74 9.93 10.18 10.03 8.29 8.93 3.98 11.12 11.32 11.39 11.13 11.14 10.60

Cr2O3 – – – 0.08 0.11 0.08 0.06 0.08 0.08 – – – 0.07 0.13 0.06 0.08 – – 0.08

FeO 10.60 11.50 9.80 9.88 10.33 13.98 12.52 11.52 11.98 12.46 11.69 16.18 11.29 12.27 11.98 11.77 14.10 13.03 10.56

MgO 17.24 16.24 17.80 17.97 16.51 13.32 14.18 15.10 14.37 15.83 15.50 12.72 18.37 13.93 14.31 14.35 12.53 13.54 15.56

MnO 0.23 0.25 0.24 0.21 0.20 0.24 0.38 0.24 0.26 0.32 0.56 0.31 0.20 0.27 0.21 0.20 0.31 0.20 0.18

CaO 12.69 12.69 12.77 12.60 10.68 11.07 10.81 10.81 11.10 10.25 9.88 10.86 10.76 11.14 11.26 11.23 11.68 11.25 11.08

BaO – – – – – – – – – – – – – – – – – – –

K2O – 0.07 – – 0.98 0.97 0.68 0.98 1.04 0.87 0.36 0.38 – 1.26 1.30 1.22 0.81 1.08 1.15

Na2O 0.16 0.20 0.15 0.17 2.10 1.88 1.54 1.89 1.73 1.25 1.51 0.57 0.18 1.85 1.93 2.01 1.57 2.21 2.03

F – – – – – – – – – – – – – – – – – – –

Cl – – – – – – 0.09 – – – – – – – – – – 0.13 –

Total 96.94 96.76 96.63 96.96 96.45 96.07 95.86 96.80 96.46 96.66 95.74 96.21 96.05 95.94 96.70 96.48 96.71 96.48 96.14

I.M.A. namesb Act Act Act Act Ed Ed Ed Ed Ed Ha M-Hbl M-Hbl M-Hbl Pa Pa Pa Pa Pa Pa

Rock type Juvenile material Lamprophyre

Grain typea MO MA OC, OIR, OOR MC MR MC MR OC OIR, OOR OOR OOR MR OOR MM MCNumber of analyses 1 1 10 2 3 2 1 2 4 1 1 1 1 1 1

SiO2 54.36 52.35 44.77 45.35 43.91 43.57 43.07 43.57 43.01 42.16 43.94 43.67 42.47 55.39 46.27TiO2 0.05 – 1.17 0.97 0.97 0.91 0.77 0.91 1.52 1.27 1.30 1.30 1.58 0.03 0.73Al2O3 1.41 2.33 9.82 9.33 10.66 11.36 11.39 11.36 11.05 10.54 10.45 10.64 11.52 1.54 8.46Cr2O3 – 0.08 0.08 0.08 0.10 0.15 0.17 0.15 0.08 – – 0.07 – – 0.08FeO 12.31 10.67 11.61 12.97 12.21 8.29 13.25 8.29 11.19 12.99 16.54 15.63 17.69 8.59 14.24MgO 16.02 16.62 15.02 13.91 14.27 16.92 12.99 16.92 14.83 13.27 10.29 10.96 9.14 18.62 13.11MnO 0.29 0.22 0.25 0.40 0.29 0.13 0.14 0.13 0.20 0.32 0.31 0.28 0.29 0.32 0.27CaO 12.89 12.47 11.12 10.95 10.74 11.14 10.95 11.14 11.28 12.26 11.90 11.83 11.69 12.58 11.54BaO – – – – – – – – – – – – – – –K2O – 0.07 1.02 0.96 0.98 1.19 1.12 1.19 1.18 0.89 0.78 0.82 0.89 – 0.72Na2O 0.10 0.22 1.76 1.47 2.03 2.03 1.83 2.03 2.07 1.97 0.98 1.07 1.10 0.18 1.25F – – – – – 0.17 – 0.17 – 0.12 – – – – –Cl – – – – – – – – – – 0.08 – – – –

Total 97.52 95.12 96.70 96.42 96.25 95.79 95.74 95.79 96.48 95.88 96.63 96.38 96.47 97.31 96.72

I.M.A. namesb Act Act Ed Ed Ed Pa Pa Pa Pa Ha M-Hbl M-Hbl Ts Act M-Hbl

a Grain type abbreviations: MM, mottled grain mantle; MOC, OZ core; MR, mottled grain rim; OIR, OZ inner rims; O

b Nomenclature of the amphiboles are according to the InAbbreviations for the other amphiboles are: Ed, edenite; H
73
O, oscillatory-zoned (OZ) grain mantle; P, pseudomorph after Hbl; MA, matrix (≤0.1 mm); MC, mottled grain core;OR, OZ outer rims; and CB, grain in contact with biotite.ternational Mineralogical Association (Leake et al., 1997). Abbreviations of common amphiboles are afterKretz (1983).a, hastingsite; M-Hbl, magnesio-hornblende; Pa, pargasite; and Ts, tschermakite.


is less common in lamprophyres, where magnesio-hornblende is prevalent. Oscillatory-zoned hornblendeis found principally in the PVB and juvenile material,where it is interpreted to be relic magmatic phenocrysts.It is unlikely that oscillatory zoning resulted fromregional metamorphism (Yardley et al., 1991; Shoreand Fowler, 1996). The oscillatory zoning is controlledby fluctuations in Si and AlIV in the T site, Mg for AlVI ,Fe2+, Fe3+ in the C site, Fe2+ and Ca in the B site, andNa in the A site. There is no regular pattern of composi-tional variation from core to rim except for an insignif-icant decrease in K and Na contents. Hornblende doesnot change its composition in contact with biotite pseu-domorphs or with biotite of the matrix/groundmass.Oscillatory-zoned hornblende does not differ in com-position from mottled zoned hornblende, or fromhornblende, which is partly replaced by fine-grainedbiotite and actinolite (Table 3). Actinolite formsmantles on, and pseudomorphs after, hornblendeand fine-grained (≤0.1 mm) matrix and groundmasscrystals.

Biotite comprises the majority of the fine-grainedmicas in the PVB and the lamprophyre, and has sim-ilar compositions in both rock types (Table 4 andFig. 10). However, some of the matrix biotites withinthe breccia have slightly higher Al in the Y site, lowerMg, and higher Na contents (up to 3.2 wt.% Na2O),thus grading into Na-biotite. Biotite does not occurin the juvenile material. The coarser biotite grains inthe PVB and lamprophyre do not exhibit any zona-t ntse assb omt evi-d titec to0t titep herT( tlyh itea

nlyi us-c Fe( en-g

Fig. 9. Plot of Si in the T site vs. Al in the C site of calcic amphi-boles from the breccia matrix, juvenile material, and lamprophyrewithin the fields of metamorphic and igneous calcic-amphibole com-positions (afterLeake, 1965). Symbols: (1) juvenile material; (2)lamprophyre; (3) breccia matrix; and (4) lamprophyre analyses of R.Barnett (personal communication 2002).

Epidote occurs as coarse or fine grains in the matrixand the groundmass of the PVB, lamprophyre and juve-nile material; we probed only coarse epidote grains.Epidote of the three rock types is similar in composi-tion, with some grains exhibiting enrichment in Cr2O3(up to 0.4 wt.%;Table 5).

The composition of the matrix plagioclase is albiteand oligoclase. Albite (Ab98An2) is present in allthree rock types, whereas oligoclase (Ab89An11) isrestricted to the PVB and the juvenile material. Oligo-clase occurs as patches within the anhedral albite grainswhich appear as complex pseudomorphs after oligo-clase. These grains are more prevalent within the juve-nile material than within the breccia.

Matrix chromite grains (≤0.1 mm) within the PVB(Table 5) contain virtually no MgO and Fe3+ (calcu-lated× Fe3+ = 0.01), little Al2O3 and high FeOTotal and

ion. In the coarse biotite, contents of all elemexcept Cr are similar to those in the matrix/groundmiotite and biotite-pseudomorphing-hornblende fr

he same rock type. We interpreted this asence for metamorphic re-equilibration of all bioompositions. Chromium is always higher (up.4 wt.% Cr2O3) in the coarse biotite (Table 4)

han in the matrix/groundmass biotite. The bioseudomorphs-after-hornblende show slightly higiO2, lower MgO and in some cases, high Cr2O3Table 4). Some of these grains have significanigher FeOTotal contents than the other biotnalyzed.

Muscovite in trace amounts (<1 vol.%) is found on the PVB and juvenile material. Some grains of movite show elevated Mg (up to 5.1 wt.% MgO) and6.5 wt.% FeO), and lower Si trending towards phite (Table 4).


Fig. 10. An Al2O3 vs. TiO2 wt.% (A) and Al2O3 vs. FeOT wt.% (B) plot of biotites in the breccia matrix and lamprophyre compared withcompositional trends for micas in kimberlite groundmass and minettes (afterMitchell, 1995a). Symbols: (1) larger biotite grains (0.1–1.5 mm)in the breccia matrix; (2) smaller grains (≤0.1 mm) in the breccia matrix; (3) biotite in the breccia matrix pseudomorphing hornblende; (4) largerbiotite grains (0.1–1.5 mm) in the lamprophyre; (5) smaller grains (≤0.1 mm) in the lamprophyre matrix; and (6) biotite in the lamprophyrepseudomorphing hornblende.


Table 4Representative electron microporbe analyses of mica compositions

Rock type Polymict volcaniclastic breccia Juvenile material Lamprophyre

Grain typea M M M M C P M M M C PNumber of analyses 3 1 1 1 16 5 3 3 10 16 10

SiO2 37.66 39.91 45.84 47.91 38.12 37.35 48.18 43.84 38.57 38.12 38.23TiO2 0.68 0.57 0.71 0.06 0.82 1.03 0.10 0.14 1.09 0.82 1.17Al2O3 16.56 18.15 17.88 30.42 16.05 16.58 29.25 28.22 15.64 16.05 16.13Cr2O3 0.17 0.07 0.09 – 0.28 0.31 0.22 0.20 0.13 0.28 0.12FeO 14.61 13.36 10.44 2.69 14.61 15.95 2.87 6.59 14.55 14.61 14.82MgO 14.47 13.77 11.03 2.24 14.61 13.10 1.87 5.14 14.43 14.61 14.10MnO 0.16 0.16 0.08 – 0.16 0.18 0.04 0.10 0.18 0.16 0.16CaO – 0.15 0.30 – – 0.08 0.05 0.08 0.34 – –BaO 0.27 0.15 0.22 0.12 0.23 0.20 0.27 0.43 0.25 0.23 0.37K2O 9.75 5.59 7.40 11.12 9.60 9.80 10.94 8.57 9.59 9.60 9.99Na2O – 1.93 3.20 0.18 – 0.05 0.14 0.15 – – –F – – – – – – – – – – –Cl – – – – – – – – – – –H2Ob 1.88 1.96 2.06 2.12 1.88 1.87 2.09 2.05 1.86 1.88 1.86

Total 96.34 95.78 99.25 96.95 96.53 96.54 96.04 95.49 96.81 96.53 97.13

Names Bi Bi Bi Ms Bi Bi Ms Ms Bi Bi Bi

a Grain type abbreviations: M, matrix grain (≤0.1 mm); C, larger grains (0.1–1.5); and P, grains pseudomorphing Hbl.b H2O determined by stoichiometry.

MnO (1.7–2.7 wt.%). Content of ZnO in the chromite,not analysed in this study, could be as high as 2–8 wt.%(Table 5), as reported byArmstrong and Barnett (2003).Chromite grains within the juvenile material and lam-prophyre are too fine-grained for EMP analysis.

9. Whole rock geochemistry

Fragment-poor volcaniclastic breccia, clast-supported volcaniclastic breccia, cognate magmaticmaterial, lamprophyre, and fragments, that may bemantle xenoliths were analysed for major and sometrace elements. The matrix of the PVB, juvenile frag-ments, and lamprophyres have a very restricted rangeof composition, with 46–50 wt.% SiO2, 2–5 wt.%total alkalies, and 9–14 wt.% MgO (Table 6). Basedon the (Na2O + K2O) and SiO2 contents, the rocksrange from alkaline to sub-alkaline. Most samples arebasic, and all are metaluminous. Variable K2O contentencompasses a range from low-K to shoshonitic series(Middlemost, 1975), and variable K2O/Na2O ratiosresult in a range from ultrapotassic to sodic composi-tions. The majority of oxides in the samples from thethree rock types do not show correlations with SiO2 or

MgO content. Only TiO2, Na2O and Al2O3 correlatenegatively with MgO, forming singular trends forall rock types. The abundances of compatible traceelements Ni, Cr and Co vary significantly, even withinone rock type (Table 6). MgO correlates strongly withNi (R2 = 0.78), and moderately with Cr (R2 = 0.48)and Co (R2 = 0.59); the best positive correlation isobserved between Ni and Cr (R2 = 0.88).

The degree to which the whole rock compositionsof the metavolcanics reflect their primary magmaticcompositions is uncertain. The low mobility of Tiin the Wawa metavolcanic rocks is manifested in itstight compositional range (∼0.4 wt.% TiO2), as deter-mined for the PVB, the juvenile material, and thelamprophyre whole-rock samples. Titanium and Al areconsidered to be least mobile elements in greenschistfacies environments (Pearce and Cann, 1973; Floyd andWinchester, 1975) and are unlikely to be modified bysubsequent alteration and metamorphism. Large vari-ations in Na2O, K2O and CaO concentrations (up to3 wt.%) in the studied samples, which do not corre-late with other major oxides are consistent with com-mon mobile behaviour of these elements in metamor-phic environments (Pearce and Cann, 1973; Floyd andWinchester, 1975; Rollinson, 1993).


Table 5Representative electron microprobe analyses of epidote, plagioclase and chromite compositions

Rock type Polymict volcaniclastic breccia Juvenile material Lamprophyre

Grain typea C CIR CR M M M C M M C CIR CB M Mb

Number of analyses 7 2 3 4 2 3 3 6 1 4 5 3 4

SiO2 37.28 37.68 37.89 67.74 64.28 0.80 37.46 68.17 66.96 37.61 37.11 37.60 68.64 0.28TiO2 0.13 0.10 0.13 n/a n/a 0.13 0.26 n/a n/a 0.02 0.41 0.34 n/a 0.21Al2O3 23.00 23.65 25.70 19.95 17.72 3.93 23.29 19.75 21.12 23.66 21.98 24.57 20.07 4.7Cr2O3 0.13 0.07 0.54 n/a n/a 40.51 0.06 n/a n/a 0.44 0.15 0.55 n/a 60.57FeO 14.03 13.50 9.96 0.32 1.61 43.84 13.84 0.29 0.37 13.20 14.97 11.47 0.25 28.88MgO 0.09 0.05 0.88 0.05 2.12 0.53 0.10 0.03 – 0.03 0.03 0.03 0.06 0.38MnO 0.15 0.09 0.08 n/a n/a 2.39 0.09 n/a n/a 0.14 0.10 0.11 n/a 1.69CaO 22.86 23.60 22.12 0.44 2.31 0.37 23.49 0.42 2.35 23.41 22.97 23.66 0.54 n/aNiO – – – n/a n/a – – n/a n/a – – – n/a 0.3Na2O – – – 11.32 9.92 n/a – 11.33 10.16– – – 11.29BaO n/a n/a n/a – – n/a n/a – – n/a n/a n/a – n/aK2O n/a n/a n/a 0.05 0.04 n/a n/a 0.05 0.08 n/a n/a n/a 0.05 n/aV2O3 n/a n/a n/a n/a n/a 0.34 n/a n/a n/a n/a n/a n/a n/a n/aZnO n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 3.31

Total 97.68 98.74 97.33 99.86 98.01 92.87 98.61 100.05 101.1 98.54 97.76 98.34 100.92 100.05

Names Ep Ep Ep Ab Olig Chr Ep Ab Oligc Ep Ep Ep Ab Chr

a Grain abbreviations: C, coarse grain (0.1–1.5 mm); CIR, coarse grain (0.1–1.5 mm) inner rim; CR, coarse grain (0.1–1.5 mm) rim; M, matrixgrain (≤0.1 mm); and CB, coarse grain (0.1–1.5 mm) in contact with biottite.

b Data fromArmstrong and Barnett (2003).c Olig.: oligoclase.

Some systematic compositional differences existbetween the PVB, its juvenile clasts and the lam-prophyres. The lamprophyres are the most potassic,exhibiting the highest K2O and BaO contents, fol-lowed by the PVB, and the juvenile material (Table 6and Fig. 11). This corresponds well to the highestmodal abundance of biotite in the lamprophyres andthe lowest modal abundance of biotite in the juve-nile material. The breccia has higher Al2O3 and lowerMgO contents than the other rock types. Among thebreccias, the clast-supported breccia shows the high-est Al2O3 and lowest MgO contents; the probableexplanation for this trend is the higher abundance ofAl- and Si-rich fragments of felsic volcanic materi-als.

We also analyzed whole rock compositions for sev-eral types of large (∼15 cm) rounded xenoliths, whichhave been completely replaced by secondary actino-lite and hornblende (Table 6). Some of the xenolithsshow extremely high contents of Cr (2300 ppm) andNi (1500 ppm), indicative of an ultramafic affinity.

10. Discussion

10.1. Igneous protoliths

The breccia matrix, juvenile material, and lampro-phyre greenschist mineral assemblages suggest a pri-mary magma of basic composition. Magmatic prede-cessors for these rocks were determined from composi-tions of relict primary minerals and preserved igneoustextures using the classification scheme ofWoolley etal. (1996). The petrographic and mineralogical crite-ria should take priority over the classification basedon the whole rock compositions, because the compo-sitions: (1) may have been modified by metamorphismand (2) are not useful in classifying hybrid diamon-diferous rocks (Rock, 1991; Mitchell, 1995b; Woolleyet al., 1996).

The coarse-grained hornblende is interpreted to rep-resent relict magmatic phenocrysts based on the oscil-latory zoning (Yardley et al., 1991; Shore and Fowler,1996) and because its compositions are consistent with

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Table 6Average abundance of major element oxides and some trace elements in the polymict volcaniclastic breccia, lamprophyre and juvenile material

Element Rock type

Polymict volcaniclastic breccia Lamprophyre Juvenile material Hbl-richxenolith

Act-richxenolith

Matrix-supported Clast-supported

10a 5a 9a 4a 1a 1a

Mean 2s(±) Range Mean 2s(±) Range Mean 2s(±) Range Mean 2s(±) Range

SiO2 (wt.%) 47.50 2.34 40.45–54.19 50.96 4.56 47.86–58.96 48.26 0.42 46.48–52.24 48.77 2.32 45.99–50.78 46.87 54.60TiO2 0.85 0.11 0.476–1.04 0.84 0.10 0.71–0.94 0.86 0.05 0.443–1.06 0.84 0.14 0.72–1.00 0.86 0.03Al2O3 11.45 0.59 9.36–12.38 12.76 1.28 10.59–13.80 9.86 0.68 3.26–11.53 9.71 2.32 7.74–12.33 9.39 2.03Fe2O3T 11.32 1.21 8.63–15.03 10.11 1.29 8.79–11.94 10.29 0.18 8.6–10.99 11.27 1.84 10.01–13.60 11.35 6.34MnO 0.18 0.02 0.13–0.24 0.16 0.03 0.11–0.20 0.17 0.00 0.151–0.215 0.21 0.05 0.18–0.27 0.23 0.16MgO 12.43 1.81 9.26–17.80 9.68 3.05 5.32–13.90 14.42 0.84 10.12–19.81 14.64 3.48 11.12–17.78 15.00 21.73CaO 8.70 1.44 6.12–11.85 9.01 1.61 6.38–10.39 8.83 0.35 7.16–12.22 9.14 0.66 8.67–9.87 10.97 12.07Na2O 1.82 0.92 0.10–4.60 2.99 0.64 2.34–3.71 1.90 0.25 0.24–3.3 1.56 1.57 0.34–3.44 1.72 0.24K2O 1.25 0.63 0.23–3.50 1.16 1.06 0.11–2.43 2.52 0.30 0.12–3.77 0.60 0.78 0.14–1.60 0.78 0.04P2O5 0.26 0.07 0.15–0.48 0.21 0.10 0.10–0.32 0.37 0.02 0.245–0.432 0.28 0.07 0.24–0.35 0.49 0.01LOI 4.48 1.82 1.17–9.94 2.43 0.95 1.44–3.54 2.66 0.22 1.29–4.25 3.33 0.25 3.03–3.53 3.18 2.57

Total 100.25 0.21 99.80–100.74 100.30 0.25 99.90–100.52 100.13 0.07 99.72–100.4 100.35 0.16 100.20–100.51 100.84 99.82

Cr (ppm) 1098.50 518.03 516–3207 620.00 261.96 244–967 1113.86 69.18 677–1600 1089.75 342.33 757–1376 1170 2349Ni 481.70 192.43 247–1252 278.60 146.86 72–481 551.14 47.44 293–916 541.75 210.24 326–734 579 1553Co 53.90 8.06 36–79 44.80 12.05 28–62 53.43 1.21 45–61 57.00 8.79 50–65 50 52Ba 381.70 146.66 60–716 352.00 346.77 19–770 585.13 64.82 229–957 196.25 320.23 22–609 170 390

a Number of analyses.


Fig. 11. A MgO–Al2O3–10K2O (wt.%) ternary plot for whole rock compositions of the polymict volcaniclastic breccia matrix, juvenile materialand lamprophyre. Symbols: (1) clast-supported breccia; (2) matrix-supported breccia; (3) juvenile material; and (4) lamprophyre.

a magmatic origin (Fig. 9). The primary magmaticcomposition of hornblende is inferred to be parga-sitic, edenitic and rarely hastingsitic, as observed incores and inner rims of the oscillatory-zoned crys-tals. These compositions, and the overall mafic char-acter of rocks reflected in their metamorphic min-eral assemblage, define their parental magma as calc-alkaline lamprophyric. The absence of pseudomorphedolivine and low contents of Ti and alkalies in relicthornblendes suggest that Group I and II kimberlites,lamproite or ultramafic lamprophyre are not viablecandidates for the magmatic protoliths (Rock, 1991;Mitchell and Bergman, 1991; Mitchell, 1995a, b).

The Wawa calc-alkaline lamprophyres originallycontained phenocrysts of hornblende and possiblybiotite and clinopyroxene. Archean lamprophyresmetamorphosed to greenschist facies have been doc-umented to retain relict igneous amphibole grains(Perring et al., 1989; Currie and Williams, 1993;Williams, 2002). Compositions of primary calcicamphiboles in the Wawa diamondiferous rocks coverthe entire compositional range of this mineral reported

for calc-alkaline lamprophyres (Rock, 1991), but alsograde to Na-rich, Al-poor calcic amphiboles (eden-ites). Evidence for the former presence of phenocrystalbiotite and clinopyroxene in the studied rocks is foundin the Cr-rich character of biotite and epidote replacingcoarse phenocrystal minerals. The epidote formed afterclinopyroxene, whereas the biotite represents pseudo-morphs after magmatic biotite with differing composi-tion resulting from element mobility and/or recrystal-lizaiton during metamorphism. None of the observedcoarse biotite grains plot on TiO2–Al2O3–FeO trendsfor magmatic micas in lamprophyres, kimberlites andlamproites (Fig. 12). This biotite is, therefore, clearlyof metamorphic origin, crystallized after high-Cr pri-mary mica phenocrysts, which are common in mantlemagmas (Mitchell, 1995a). It is unlikely that the coarseCr-rich biotite is replacing Cr-poor magmatic horn-blende because Cr is immobile during metamorphism(Rollinson, 1993). Moreover, fresh clinopyroxene andbiotite are described from Archean calc-alkaline lam-prophyres in the Abitibi greenstone belt 400 km east ofWawa (Wyman and Kerrich, 1993).


Fig. 12. The Al2O3 vs. TiO2 (A) and MgO vs. SiO2 (B) plots for whole rock compositions of the Wawa calc-alkaline lamprophyric rocks. Fields1–7 outline compositional fields for lamprophyres and other primary diamondiferous rocks (Scott-Smith, 1995; Mitchell, 1995a; Rock, 1987,1991) and include alkaline lamprophyres (1), calc-alkaline lamprophyres (2), ultramafic lamprophyres (3), (4) Wawa kimberlites (Kaminskyet al., 2002), (5) Groups 1 and II kimberlites, (6) lamproites, (7) olivine lamproites. Symbols: 1—matrix-supported PVB; 2—clast-supportedPVB; 3—juvenile material; 4—lamprophyre; 5—Archean shoshonitic lamprophyres (Wyman and Kerrich, 1993); 6—Archean Uchi subprovincecalc-alkaline lamprophyres (Wyman and Kerrich, 1989); 7—Archean Wabigoon and Wawa subprovince calc-alkaline lamprophyres (Wymanand Kerrich, 1989); 8—Archean Abitibi subprovince calc-alkaline diamondiferous lamprophyres (Williams, 2002); and 9—Archean Yilgarncalc-alkaline lamprophyres (Currie and Williams, 1993).


Compositions of chromites in the metavolcanicWawa rocks (Table 5andWilliams, 2002) are in therange typical for lamprophyres but are dissimilar tothose associated with kimberlites and lamproites. Calc-alkaline lamprophyres often contain chromites that arelow in MgO and high in ZnO (up to 7 wt.%) and MnO(up to 4 wt.%;Rock, 1991).

Whole-rock major-element geochemistry of theWawa metavolcanic rocks supports their interpretationas metamorphosed calc-alkaline lamprophyres. TheAl2O3–TiO2 and MgO–SiO2 diagrams show that themajority of the samples plot in the calc-alkaline field,occupying the same range as other known Archeanlamprophyres, many of which have been classified ascalc-alkaline (Wyman and Kerrich, 1993; Fig. 12). The2.67 Ga lamprophyres of the Abitibi belt are very sim-ilar to Wawa lamprophyres petrographically and geo-chemically, although more K-rich in character. Onlysome of these Abitibi and Wawa lamprophyric dykesand breccias are diamondiferous. The barren lampro-phyres show no major petrographic contrast to thediamondiferous counterparts, although the latter have agreater abundance of chromite and higher Mg-numbersof whole rock compositions (Williams, 2002). Mg-numbers of the diamondiferous lamprophyres (75–85;Williams, 2002) are higher than those of primitivemantle magmas, suggesting a contamination by mantleperidotite, as is typical for other diamond-bearing vol-canic rocks. The lamprophyric magmas of Wawa wereclearly sourced in the mantle as evidenced by their highM ,e rtedb lam-p enceo iths(

1b

ostr ag-m tionc sesa ity.T sivea

1. A repeated occurrence of conformable PVB bedsthroughout the stratigraphic sequence where theyare intercalated with local volcanic rocks. The bedscan be traced for∼4 km along the strike of localstratigraphic units.

2. Large sizes of individual breccia occurrences, whichgreatly exceed possible diameters of volcanic dia-tremes (300–3000 m).

3. Sedimentary structures, such as massive and coarsetail normal-graded beds, slump structures of foldedsemi-consolidated material. Diatreme breccias, incontrast, are massive and cannot be bedded, sortedor graded (Field and Scott-Smith, 1999).

We compare characteristics of PVB to those offragmented volcaniclastic rocks of various origins(pyroclastic flow, pyroclastic fall, pyroclastic surge,fluvial flow, grain flow, turbidity current, slidesand debris avalanches, and debris flows;Table 7).We interpret PVB to represent lahar deposits, i.e.debris flows rich in a volcanic component (Cas andWright, 1988; Vallance, 2000), based on the followingevidence.

1. The volcaniclastic breccia is generally massive andstructure-less, unlike most types of volcaniclasticdeposits (Table 7). The rare sedimentary structuresseen in Wawa PVB’s are massive and coarse tailnormal graded beds (Shultz, 1984; Cas and Wright,1988; Boggs, 1995). The crude normal grading may

port

2 essousasar-

vials

icaloes

3 ticlesitsto

vi-res-ruleept

gO, Ni, Cr and Co contents (Table 6) in all samplesxcept for the crustally contaminated clast-supporeccia. A deep mantle source for the calc-alkalinerophyres is unequivocally supported by the presf diamonds and mantle-derived ultramafic xenolMorissette and Francis, 2004).

0.2. Volcanology of the polymict volcaniclasticreccias

The breccias have a volcanic origin as they hare lapilli and bomb-sized juvenile and cored frents, which are interpreted as pyroclasts. Breccia

an result from several drastically different processsociated with intrusive or explosive volcanic activhere is abundant evidence that PVB’s are not intrund do not occur in diatremes, including:

have resulted from particle settling during trans(Smith, 1986) or kinetic sieving (Vallance, 2000).

. The breccia units are thick (maximum thickn∼110 m) and have been found in discontinuregions over a 5 km× 10 km area; such large areoccupied by thick volcanogenic units are unchacteristic of most pyroclastic deposits, and fluand grain flows (Table 7). In contrast, large volumeof material and long outflow distances are typfor debris flow deposits that originate on volcan(McPhie et al., 1993).

. The breccia is poorly sorted with respect to parsize and density, unlike most pyroclastic depo(Table 7). Fragment sizes range from sandmegablock (up to 9 m in size). There is little edence of sorting on the basis of density. The pence of accidental megablocks of country rockout most types of volcaniclastic deposits exc

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138(2005)57–87

Table 7A comparison of the Wawa volcaniclastic breccia with known volcaniclastic deposit typesField characteristics Wawa volcaniclas-

tic brecciaPyroclastic flowa Pyroclastic falla Pyroclastic surgea Fluvial flowa Grain flowa Turbidity currenta Slides and debris

avalanchesaDebris flowa

Bedding + + + − − − + +Commonly absent.Rare, 30 cm to >1 mbedding

Rare but may haveinternal layering(<0.1 to 100 mthick)

Massive, laminatedand plane parallelbeds (cm to >10 mthick), or absent

Massive or planarbeds (proximally1 m to distally1–10 mm thick) orabsent

Present (3–10 cmthick), or absent

Internally diffuse bedswith steep primary dip(cm to >10 cm thick)

Absent or planarbeds, laminated,diffuse layering (cmto >10 cm thick)

Absent or may be dif-fusely layered

Rare or may haveup to >1 m internallayering

Other structures − − − − − − + +Slump structures,sedimentary dikes,jig saw fit texture

Anisotropicfabric, gas seg-regation pipes,high-temperaturedevitrificationtexture

Commonlyisotropic fabric,bomb sags

Bomb sags, cross-beds. chute-, pool-,flame- and slump-structures

Beds: cross-, planar,or laminated, andtractional structures

Beds: massive, internallayered, or cross-stratification

Upward fining bedthickness/grain sizeprofiles; cross-beds,slump, dish and pil-lar structures

Soft sediment deforma-tion, jig-saw fit texture

Jig saw fit texture,vesicles, slumpstructures, com-monly isotropicfabric

Grading + + + + − + − +Commonly absent,may be crude nor-mal, reverse, or mul-tiple

Rare may be crudenormal, reverse, ormultiple

Reverse common,may also be normaland absent

Rare may be crudenormal, reverse, ormultiple

Commonly normal Reverse grading com-mon

Absent; crude orwell developed nor-mal, reverse, multi-ple

Absent Commonly absent,may be normal,reverse, multiple

Unit thickness + + + + − + + +10 nm to >100 m cm to 10 nm cm to 10 nm Proximal >10 m and

distal 1–10 cmup to 10–100 m 10–100 cm m to >100 m m to >100 m 1 m to 10–50 m

Distribution + + + + − + + +>100 m to sev-eral km

km to 90 km km to 90 km Several km to 10 km Confined to valleys Localized, <0.1 km 10 m to 90 km Up to 90 km from source 10 m to 90 km

Volcanological sort-ing

+ − − + + + + +

Poor Poor Moderate to good Moderate Poor to good Poor Poor to good Poor Poor

Large boulders(>2 m)

− − − − − − + +

Present Absent Absent Absent Absent Absent Absent Present Present

Common fragmentangularity

+ + + − + + +

Dominantly angular Dominantly suban-gular

Angular Angular Rounded to sub-rounded

n/ab Angular to rounded Angular Angular tosubangular

Composition − − − + + + + +Polymict Monolithic, juve-

nile pyroclastsTypically monomicttephra

Dominantlymonomict

Polymict andmonomict

Polymict and monomict Polymict andmonomict

Polymict and monomict Polymict

a Characteristics of the Wawa breccia deposits that are similar [+] or different [−] to the volcaniclastic deposits ofCas and Wright (1988), McPhie et al. (1997),Fisher and Schmincke (1984).b n/a: not found in the literature.


debris flows, and debris avalanches and slides(Table 7).

4. Good preservation of angular clasts, cored lapilli,juvenile magmatic material, unbroken phenocrysts,and jigsaw-fit textures indicates that the depositshave not been significantly reworked by epiclas-tic fluvial environments (Table 7). The preserva-tion of delicate features is typical of debris flowsbecause transport occurs by laminar flow, and par-ticles are protected by the cohesive strength ofthe matrix (Johnson, 1970; Fisher and Schmincke,1984; Vallance, 2000). Mafic volcanic fragments,which would be prone to mechanical disaggregationand chemical alteration in turbulent environments,are also well preserved in the PVB.

5. Fragment types within the breccia represent a widerange of lithologies (at least eleven different types;Table 2) atypical of pyroclastic deposits (Table 7).The presence of many different types of fragmentsis evidence for significant transport (McPhie et al.,1993).

6. The matrix of the breccia is mineralogically similarto the pyroclastic component of the calc-alkalinelamprophyric composition, and must have formedfrom the lamprophyric volcanic ash.

Several features observed in the PVB are easilyexplained under a mass-failure flow origin, even thoughthese characteristics need not be present in all debrisflows. Thin irregular tabular bodies of PVB foundw byb heym e-fi redb na oset ene res.F s asrb ruc-ta

owe beds( int -c vol-

caniclastic kimberlites (Field and Scott-Smith, 1999;Graham et al., 1999), which often contain previouslyformed “volcaniclastic autoliths”. The dominance ofdifferent lithological fragment types in highly localizedareas within the breccia also supports this conclusion.Such a pattern suggests varied sources for the mass-flow material. The material may have been depositedby individual debris tongues separated spatially or tem-porally, or by a single flow that sampled many differentlithological units and deposited their fragments locally(Vallance, 2000).

10.3. Tectonic setting of the calc-alkalinelamprophyric magmatism

The calc-alkaline lamprophyric magmas, whichformed intrusive dykes and associated volcaniclas-tic breccias were emplaced episodically from 2.66 to2.73 Ga as a result of subduction-driven melting andmagmatism in the Michipicoten Greenstone Belt. Theepisodic nature of magmatism is inferred by: (1) theoccurrence of at least two beds of breccia at differentlevels in a continuous stratigraphic sequence (Fig. 4C);(2) the complex contact relationships between thebreccias, the lamprophyres, and the local supracrustalrocks; (3). The wide range of ages reported for the brec-cias and lamprophyres. Dates for breccias vary between2727 and 2670 Ma, whereas lamprophyre dykes wereslightly younger, being emplaced between 2715 and2664 Ma.

undc ma-t ec-c altst ithm on-t pro-p nicm eo-l ing( ricm tru-s atismo ral,l dja-c valw ,2

ithin the country rock and large boulders infilledreccia matrix may represent sedimentary dykes. Tay have been formed by forceful injection of liqu

ed material into fractures in adjacent rocks triggey slumping (Boggs, 1995). Breccia inclusions withidjoining intermediate to mafic intrusive rocks cl

o the contacts (Fig. 5C) are interpreted to have bemplaced in the same fashion as the infilled fractuolded semi-consolidated material which appearare large blocks within the breccia (Fig. 5J) coulde an example of slump, soft-rock deformation st

ures commonly associated with debris flows (Smithnd Lorenz, 1989; Boggs, 1995).

The PVB formed as a result of multiple mass-flvents, as indicated by the presence of severalBoggs, 1995) and fragments of older breccias withhe younger breccia (Fig. 5A). Similar repetitive proesses of mass-flow deposition are reported for

The calc-alkaline lamprophyric magmas are fooeval with pre-tectonic and late tectonic magism of the Michipicoten Greenstone Belt. Brias formed simultaneously with calc-alkaline baso rhyolites, as they are found intercalated wafic flows with conformable upper and lower c

acts. Coeval development of some of the lamhyric magmas with post-collisional late orogeagmatism is evidenced by a number of g

ogical observations. Structures of magma mixFig. 8D) document emplacement of lamprophyagma into unconsolidated intermediate-mafic in

ives, suggesting near-contemporaneous magmf the two compositional types. There is structu

ithologic and geochronological evidence that aent breccias in the Lalibert-township were coeith the 2673 Ma late orogenic syenites (Stott et al.002).


The tectonic setting of Wawa lamprophyres is typ-ical for calc-alkaline lamprophyres in general. Theyoccur in subduction zone environments in associationwith late orogenic granitoids or with shoshonitic suitesinboard from the volcanic front, and less frequentlywith calc-alkaline volcanics (Rock, 1991; Winter,2001). The worldwide summary of magmatic associa-tions observed for calc-alkaline lamprophyres (Rock,1991) lists 16 occurences where they are found togetherwith basalt–andesites–rhyolite suites. These occur-rences, in Tasman Fold belt, Indonesia, Mexican Vol-canic Belt, Papuan Arc, North and South AmericanCordillera, and the Mediterranean, belong to bothancient and present-day convergent margin environ-ments. The modern analogue for the geology andmagmatism of∼2.7 Wawa subduction zone could bewestern Mexico. There, an association and a transitionbetween Quarternary to Pliocene volcanic calc-alkalineand shoshonitic lamprophyres and K-rich andesitesand basaltic andesites are reported (Carmichael et al.,1996; Allan and Carmichael, 1984). Local extensionalgraben tectonics superimposed on the active conti-nental subduction setting creates a diverse suite oflamprophyric–andesitic volcanism (Luhr et al., 1989;Carmichael et al., 1996). For the Pliocene lampro-phyres, the combination of extension and subduc-tion resulted from oblique subduction and forma-tion of a passive pull-apart basin (DeMets and Stein,1990).

We suggest emplacement of the Wawa lampro-p ctiveA uth-e cin-d aele sf hedo re-s stic,fl m-p oxi-m ria-f ,1

aveb n ofd thata er-l bod-

ies accelerated by high volatile contents in the magmas.Similar high rates of ascension are expected in volatile-rich lamprophyres, which are emplaced explosively.These should have higher diamond potential than morecommon lamprophyric bodies intruded non-violentlyas dykes. This is consistent with observations in thestudy area, where the younger Wawa lamprophyres areless diamondiferous than the Wawa epiclastic breccias,even though the latter have been diluted by abundantcountry rock fragments. Pyroclastic deposits not mod-ified and diluted by secondary epiclastic processesshould have the highest diamond potential among calc-lamprophyric source rocks.

11. Conclusions

1. Two main types of Late Archean diamondifer-ous rocks are recognised in Wawa (south-westernSuperior Province): metamorphosed polymict vol-caniclastic breccia, and lamprophyre. The lam-prophyre occurs is 0.5–3 m dykes, whereas thebreccia forms thick, 60–110 m beds traceable inintermittent outcrops along strike for more than4 km. The lamprophyre dykes are younger than thebreccia.

2. The breccia and the lamprophyre are metamor-phosed to greenschist facies, and consist of actino-lite, hornblende, chlorite, albite, biotite, with acces-sory titanite, epidote, and quartz. The only relict

ry-r ofition

strylith

3 asvol-

ies,anddi-

4 em-nd

pi-sodi-ion

hyric magmas in local small-scale grabens in an archean subduction zone. As in Mexico and sorn Alberta, where lamprophyres are found iner cones (Allan and Carmichael, 1984; Carmicht al., 1996; Kjarsgaard, 1994), Wawa lamprophyre

ormed pyroclastic rocks. These rocks were wasut in lahars to form thick epiclastic deposits perved as massive breccia beds. Similar epiclauvially reworked lahar deposits of minette coositions, as well as mass flow deposits pral to the vent and 50 Ma well-preserved sco

all, are descibed in southern Alberta (Kjarsgaard994).

The Wawa calc-alkaline lamprophyres must heen emplaced very rapidly to ensure preservatioiamond in resorbing magma. It is not accidentalll major primary diamondiferous rocks, i.e. kimb

ites and lamproites, are fast-emplaced magmatic

magmatic mineral is coarse-grained oscillatozoned hornblende. The overall mafic charactethe rocks, preserved igneous textures, composof relict hornblende, and whole-rock geochemisuggests that composition of the magmatic protofor these rocks is calc-alkaline lamprophyres.

. The polymict volcaniclastic breccia formeddebris flows (lahars), as suggested by its largeume, stratigraphy, wide range in clast lithologpoor sorting, the presence of delicate structureslarge country-rock boulders, and paucity of sementary structures.

. The Wawa lamprophyric magmas formed contporaneously with felsic to mafic volcanic rocks alate orogenic intrusives of cycle 3 of the Michicoten greenstone belt, and were emplaced epically from 2.7 to 2.67 Ga in local areas of extensin an active Archean subduction zone.


Acknowledgements

We are grateful to Band-Ore Resources Ltd. fortheir support and financial contribution to the project.This research was partly funded by a National Scienceand Engineering Research Grant and NSERC Indus-trial Post-Graduate Scholarship to N. Lefebvre. We areindebted to H. Grutter for his tremendous foresight andhelp in pursuing this exciting project. Field work wascarried out with the invaluable assistance of B. Duessand D. Smithson. Also we wish to thank B. Kjars-gaard, B. Doyle, K. Hickey, G. Dipple, K. Russell, andM. Raudsepp for their discussions and comments. Weappreciate reviews of the manuscript by Drs. Thurston,Ayers and Kaminsky and the editorial assistance of K.Breitsprecher.

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