Precambrian Research 101 (2000) 277 – 311
Evolution of a submerged composite arc volcano:
volcanology and geochemistry of the Norme´tal volcanic
complex, Abitibi greenstone belt, Que´bec, Canada
Benoit Lafrance
a,*, Wulf U. Mueller
a, Re´al Daigneault
a, Normad Dupras
baDe´partement des Sciences de la Terre,Uni6ersite´ du Que´bec a` Chicoutimi,Chicoutimi Que.,Canada G7H2B1 bFalconbridge Limited,La6al Que.,Canada H7L5A7
Abstract
The 4 km-thick Archean Norme´tal volcanic complex (NVC), composed of basaltic andesite, dacite, and rhyolite, is represented by five distinct volcanic phases and one sedimentary phase. Initial volcanic construction features effusive mafic volcanism characterized by massive, pillowed and pillow breccia flows and local massive dacite (phases 1 and 2a). Prominent felsic volcanism of phase 2 commences locally with tuffs, lapilli tuffs and lapilli tuff breccias derived either from hydroclastic or autoclastic fragmentation processes (phase 2b). The principal constructive phase of the NVC (phase 2c) is composed of pillowed andesite, massive dacite, and dominant massive, flow banded and lobate rhyolite flows. Autoclastic or hydroclastic brecciation of the former have produced rhyolitic tuff, lapilli tuff and lapilli tuff breccia. Rhyolitic volcanism continued with eruption of lava flows (phase 3) and the intrusion of dykes and felsic endogenous domes (phases 3 and 4). A subsequent 20 – 70 m-thick sedimentary unit, composed of volcaniclastic turbidites and pelagic background sediments, constitutes a marker horizon indicating volcanic quies-cence. Renewed volcanism of phase 5 is characterized by mafic to felsic turbiditic lapilli tuffs and tuffs, and mafic to felsic flows or intrusions. The felsic lapilli tuffs, tuffs and flows host the Norme´tal VMS deposit. The geometry and volcanic stratigraphy of the NVC suggests emission of viscous, phenocryst-rich felsic flows from three principal centers, including a parasitic western vent, the major central 6 km-wide cauldron structure and an eastern vent. Voluminous viscous felsic lava over a large area supports the inference of numerous vents whereby individual centers coalesced to produce a composite or complex stratovolcano. Proximal to distal facies changes, variable rhyolitic unit and lobe closures argue for multiple conduits. The VMS deposits are located at the western edge of the central cauldron. Geochemical analyses show two complete compositional spectrums (phases 1, 2, 4 and 5) from basaltic andesite to rhyolite. The Zr/Y and LaN/YbNratios of phases 1, 2, 4 and 5 show a transitional affinity whereas phase
3 is tholeiitic to slightly transitional. Multi-element diagrams suggest that all phases are consistent with subduction-re-lated processes. The mafic-felsic NVC, a composite volcano that formed upon a shield type volcano, displays subaqueous effusive dominant volcanic construction at depth below storm wave base, as indicated by pillowed flows, turbiditic and pelagic sedimentary rocks, and massive sulphide deposits. Geochemistry and physical volcanology of the NVC are consistent with construction of an immature arc volcano. The submerged Izu-Bonin arc volcanoes may
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* Corresponding author. Fax: +1-418-5455012.
E-mail addresses:benoit –lafrance@uqac.uquebec.ca (B. Lafrance), wmueller@uqac.uquebec.ca (W.U. Mueller)
be modern analogues. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Archean; Mafic-felsic volcanism; Subaqueous; Volcanic facies; Composite arc volcano; Geochemistry
1. Introduction
The majority of volcanogenic massive sulphide deposits (VMS) in the geologic record are associ-ated with rhyolitic rocks (Rona, 1988; Barrie and Hannington, 1997), whereas modern VMS coun-terparts are observed mainly in mid-ocean ridge spreading centers which are generally basaltic in composition (Rona, 1988). Only recently has there been a shift in research emphasis to VMS mineral-ization in modern arc-backarc systems (Rona and Scott, 1993; Ishibashi and Urabe, 1995). Studies
on active mafic/felsic and felsic submerged
arc-backarc settings are restricted to submersible dives, such as the Sumisu Rift (Smith et al., 1990), the Myojin knoll (Iizasa et al., 1999), the Lau (Fouquet et al., 1993) or Mariana backarc basins (Lonsdale and Hawkins, 1985) as well as the Kermadec arc volcanoes (Wright et al., 1998). In contrast, ancient arc sequences (Sato and Amano, 1991; Kano et al., 1993; Martin-Barajas et al., 1995) are important windows for the study of modern subaqueous arc-forming processes. The Abitibi Subprovince, a well documented Archean greenstone belt, is composed of numerous felsic or mafic/felsic volcanic edifices (Chown et al., 1992), which favors in-depth volcanological analyses of subaqueous felsic volcanic complexes related to VMS deposits. Understanding the physical vol-canology of submerged volcanoes is important for establishing eruptive mechanisms, mode of vol-canic emplacement and geometry of volvol-canic edifices, as well as defining proximal and distal volcanic facies with respect to VMS deposits.
Characteristics of felsic and mafic Archean lava flows have been addressed in detail (Dimroth, 1977; Dimroth et al., 1978; de Rosen-Spence et al., 1980; Cousineau and Dimroth, 1982), but little emphasis has been placed on the evolution of entire subaqueous volcanic complexes (Gibson and Watkinson, 1990; Morton et al., 1991). The principal aim of this study is to document the volcanic construction of the mafic to felsic
Norme´tal volcanic complex (NVC) in the Abitibi greenstone belt. A two-fold approach based on regional and detailed mapping, sampling and structural analysis was conducted emphasizing: (1) physical volcanology; and (2) geochemistry. In addition, the Norme´tal volcanic complex is com-pared to other volcanic centers in the Abitibi belt and modern arc volcanoes in order to constrain Archean arc models.
2. Geological setting
2.1. Abitibi greenstone belt and regional geology
The 300×700 km Abitibi greenstone belt, the
largest coherent Archean supracrustal sequence in the world, hosts numerous volcanic centers with economically important VMS deposits (Franklin et al., 1981; Chartrand and Cattalani, 1990). Chown et al. (1992) divided the Abitibi belt into northern (NVZ) and southern volcanic zones (SVZ; Fig. 1) based on the work of Dimroth et al. (1982), U-Pb age determinations (Mortensen, 1993a,b), stratigraphic and structural relation-ships (Daigneault and Archambault, 1990), geo-chemical signatures (Ludden et al., 1984; Picard and Piboule, 1986a,b), and sedimentary basin analysis (Mueller and Donaldson, 1992). The NVZ is composed of volcanic cycle 1 (2730 – 2720 Ma), representing an extensive, subaqueous
primi-tive basalt plain with dispersed mafic/felsic and
felsic complexes, which collectively define a dif-fuse and incipient arc. Volcanic cycle 2 (2720 – 2705 Ma) is consistent with a mature and dissected volcanic arc. Mueller et al. (1996) di-vided the Abitibi greenstone belt into four distinct periods or tectonic stages: (1) arc formation and construction (2730 – 2698 Ma); (2) arc – arc colli-sion (2696 – 2690 Ma); (3) arc fragmentation (2689 – 2670 Ma); and (4) arc exhumation (2660 – 2640 Ma). The Norme´tal volcanic complex
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resenting the arc formation and construction pe-riod, is one of the least known mafic/felsic centers belonging to volcanic cycle 1 of the NVZ (Fig. 1). The study area is divided into north and south Norme´tal blocks (Fig. 2), with the northern block composed of (i) basalt and iron formation of the Gale Group (Latulippe, 1976); and (ii) the Norme´tal volcanic complex (NVC). Basalt, andes-ite, iron formation and gabbro constitute the south Norme´tal block (Pe´loquin, 1994). The con-tact between the NVC and the south Norme´tal block is the Norme´tal deformation zone, charac-terized by pure shear with local dextral reactiva-tion (patten fault; Fig. 2; Lafrance et al., 1996). The good rock exposure, principally in the west-ern part of the NVC, coupled with the limited influence of deformation outside the regional fault corridors, enable good recognition of volcanic textures and estimation of unit thickness and lat-eral extent. Elsewhere, industry diamond drilling
and geophysical surveys compensate for scarcity of outcrops. Greenschist facies metamorphism is prominent in the NVC whereas upper greenschist facies (biotite isograd) and amphibolite facies
metamorphism is observed only near the
Rousseau and Patten plutons. A metamorphic
hydrothermal assemblage of quartz+sericite+
chlorite+carbonate+chloritoid9tourmaline is
characteristic of the study area (Teasdale, 1993; Pe´loquin, 1994). A ubiquitous semi-concordant
carbonate9chloritoid alteration zone is
consis-tent with a Mattabi-type VMS deposit (Morton and Franklin, 1987). Precise plutonic U-Pb zircon
age determinations yielded 2716 + / −3 Ma for
the Val-St-Gilles pluton (Vaillancourt and
Machado, 1995), 2710 + / −2 Ma for the
Norme´tal pluton (Zhang, in Pe´loquin, 1994), and 2703 + / −2 Ma for the Rousseau pluton (Davis et al., 1992).
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Fig. 3. Schematic stratigraphic column of the Norme´tal vol-canic complex and adjacent units. The Norme´tal volvol-canic complex includes the 4 km-thick sequence of mafic-felsic vol-canic rocks, the Norme´tal sedimentary rocks and the synvol-canic Norme´tal and Val-St-Gilles plutons. Legend as in Fig. 2.
Norme´tal mine (11 M/t grading 5.12% Zn, 2.15%
Cu, 0.549 g/t Au and 45.25 g/t Ag; Teasdale,
1993) and the satellite Normetmar deposit
(160 000 t at 12.6% Zn), as well as associated volcanic rocks and intrusions of phase 5, are herein referred to as the Mine Sequence (Figs. 2 and 3).
3. Volcanology of the Norme´tal volcanic complex
3.1. Characteristics and terminology
The NVC was mapped at scales of 1:5000, 1:1000 and 1:500. For the following descriptions, the NVC is divided geographically into western (Fig. 4), central (Figs. 5 and 6) and eastern seg-ments (Fig. 7), and each segment is separated by major Proterozoic diabase dykes (Fig. 2). Each segment has a specific local stratigraphy and a distinct volcanic evolution (Fig. 2). Outcrop loca-tions correspond to the volcanic facies symbols plotted in Figs. 4 – 7. Felsic units are defined using phenocryst type, percentage and size (Table 2). With the exception of quartz-feldspar-porphyry-3 (Qfp-3), which only occurs in phase 4, felsic rock types are not restricted to a specific volcanic phase (Table 1).
The terminology employed is based on the defi-nition of Fisher (1961) whereby volcaniclastic rocks are ‘deposits composed predominantly of volcanic particles’. Volcaniclastic deposits are de-scribed using the standard granulometric classifi-cation of Fisher (1961, 1966), which is purely
descriptive and non-genetic. The general
terms volcanic breccia (clasts \2 mm) and
vol-canic sandstone (grain size B2 mm) are
em-ployed for the volcaniclastic sediments and rocks of uncertain origin (Fisher and Schmincke, 1984; pp. 91 – 92). Tuff, lapilli tuff and lapilli tuff
brec-cia clearly associated with lava flows are
referred to as hyaloclastites (McPhie et al., 1993). The term volcanic phase referred here to a distinct period of volcanic construction at the
edifice scale, by opposition to volcanic
cycle which referred to a belt scale (Chown et al., 1992).
2.2. Norme´tal 6olcanic complex (NVC)
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Table 1
Volcanic phases of the Norme´tal volcanic complex (NVC)a
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3.2. Phase 1:basaltic andesite dominated subaqueous 6olcanism
The initial phase of volcanism is represented by a 1 – 2 km-thick sequence of basaltic andesite, andesite and minor dacite. The basaltic andesite and andesite are represented by massive, pillowed and minor pillow breccia flows. Flows are locally porphyritic, with up to 3% 1 mm-feldspar and contain 10 – 30% amygdules, 0.3 – 2 cm in diame-ter. Pillowed flows are 4 – 20 m thick with 0.5 – 2 m-wide pillows with local concentric cooling joints. The pillow breccias are composed of 10 – 30 cm-amoeboid pillow fragments. Pillows in the
western, central and eastern segments of the NVC display a south younging direction (Fig. 2). Local
interstratified dacite flows are massive and
aphanitic.
3.2.1. Interpretation
Massive and pillowed flows suggest effusive volcanism in a subaqueous environment (Dimroth et al., 1978; Cousineau and Dimroth, 1982), whereas the lateral extent of the mafic unit is consistent with a subaqueous ocean floor setting (Chown et al., 1992). The massive dacite remains problematic and it may represent either a flow or intrusion.
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Fig. 7. (A) Volcanic facies and lithological map of the eastern segment of the Norme´tal volcanic complex (see Fig. 2 for location). Location of volcanic facies symbols corresponds to outcrop location. (B) Outcrop map of Qfp1 lapilli tuff and laminated tuff intruded by aphanitic rhyolite lobes and diorite. Large arrow indicates prominent flow direction.
Table 2
Petrographic distinction of the Norme´tal volcanic complex rhyolites
Aphanitic Qfp1 Qfp2 Qfp3 Qp1
Quartz Quartz and
Microfeldspar
Phenocryst typea Quartz and feldspar Quartz and feldspar
phyric feldspar
– B5% 10–25%
Quartz phenocryst B5% 10–25%
percentage
0.5–1 mm 1–2 mm 3 mm–1 cm
Quartz phenocryst size – 1–2 mm
aFrom macroscopic and thin sections observations.
3.3. Phase 2:andesitic-dacitic and rhyolitic subaqueous 6olcanism
The 0.8 – 2.2 km-thick andesite-dacite and rhyo-lite of phase 2 (Fig. 2), the principal constructive phase of the NVC, is divided into: (i) a basal andesite unit (phase 2a); (ii) a medial felsic vol-caniclastic unit (phase 2b); and (iii) an uppermost andesite-dacite and rhyolite unit (phase 2c) which is distributed in the western (Fig. 4), central (Figs. 5 and 6) and eastern (Fig. 7) segments.
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The 5 – 50 m-thick felsic volcaniclastic deposits of phase 2b are constrained to the western (Fig. 4) and central segments (Fig. 6). Western segment
deposits are characterized by basal tuff overlain by lapilli tuff breccia (Fig. 8b). The 1 m-thick fine to coarse tuff contains 0.5 – 20 cm-thick massive,
Fig. 8. Volcanic facies of phases 2a and 2b of the western segment of the NVC (see Fig. 4 for location). White arrow indicates younging direction. Pencil (15 cm) and hammer (35 cm) for scale. (A) Stratigraphic top indicated by pillows of phase 2a andesite. (B) Volcaniclastic deposit of phase 2b which are characterized by 1 m-thick turbiditic tuff (Tt) at the base overlain by 2 – 10 m-thick matrix-supported lapilli tuff breccia (Ltb). (C) Fine to coarse basal turbiditic tuff of phase 2b, characterized by massive graded Ta
beds laminated Tbbeds and convoluted Tcbeds. (D) Details of load cast structure (Lc) present at the base of a Tabed that overlies
convoluted Tcbed. (E) Matrix-supported lapilli tuff breccia of phase 2b. Subrounded aphanitic and vesicular fragments could be
derived from the underlying andesite of phase 2a. Uppermost Tcbed of the basal turbiditic tuff is observed at the bottom of the
Fig. 9. Characteristics of the eastern part of the central seg-ment of the NVC (see Fig. 10 for location). Pencil (15 cm) and hammer (35 cm) for scale. (A) Subrounded aphanitic (Aph) and Qfp2 fragments in the lapilli-tuff breccia of phase 2b. (B) A 5 – 10 m-thick Qfp1 sill of phase 2c with east closing endogenic 2 m-wide lobe intruding Qfp2 lapilli tuff breccia of phase 2b. Top of photograph to the east. (C) Details of the chilled margin (Cm) in the Qfp1 sill at the contact with Qfp2 lapilli tuff breccia.
tuff breccia, 2 – 10 m-thick, containing 10 – 60 cm subangular to subrounded fragments (Fig. 8e) as well as 2 – 20 m-thick, clast-supported lapilli tuff breccia with 2 – 8 cm-angular to subrounded Qfp2 fragments (Fig. 8f), follow up-section. Phase 2b of the central segment is characterized by 20 – 30 m-thick Qfp2 massive clast-supported lapilli tuff breccia (Figs. 6 and 10) composed of abundant subrounded 5 – 20 cm Qfp2 clasts (80%), similar to the matrix, and aphanitic clasts (20%; Fig. 9a). The lapilli-tuff breccia has a sharp depositional contact with the andesite-dacite and rhyolite flows (Fig. 10), which marks the presence of a synvol-canic fault.
Phase 2c, 0.8 – 1.5 km thick, is characterized by andesite-dacite and rhyolite flows, dykes and in-trusions. An example of the 90 – 580 m-thick mas-sive and pillowed andesite unit associated with pillow breccia and lapilli tuffs and sills is well exposed in the eastern part of the central segment (Figs. 6 and 11). Two 2 – 25 m-thick massive flows grade up-section into 2 – 6 m-thick pillowed flows (Fig. 12a, b), 2 m-wide master tubes, or 5 – 23 m-thick pillow breccias with amoeboid fragments (Fig. 12c). A 4 m-thick lapilli tuff (Fig. 12d) with subangular to subrounded clasts follows up-sec-tion. Minor dacites are massive with local, angu-lar breccia-size fragments.
The 150 – 600 m-thick felsic units of phase 2c are flows, dykes and intrusions with phenocryst variations permitting identification of individual flow units (Table 2). Associated fragmental de-posits (tuff, lapilli tuff, and lapilli tuff breccia) are composed of angular to subrounded clasts (Table 1). Columnar jointing, 5 – 20 cm in diameter is locally observed in massive flows or intrusive bod-ies. Lateral flow facies transitions over a distance of 1 – 2 km are observed in felsic flow units of the western segment. Typically, the flow units display a change from massive to well defined, 1 – 20 m-thick, west-closing lobes with massive centers and a marginal metre-thick flow-banding to tuff, lapilli tuff breccia and laminated tuffs (see flows labeled a-b-c in Figs. 4 and 5). Large domal massive bodies, characterized by uniform phe-nocryst content, massive facies and local intrusive contacts were also identified (Fig. 4).
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Chronological and geometrical relationships of felsic units defined by facies mapping and phe-nocryst variation reveal lava lobes and complex unit shapes. For example, a chronological rela-tionship is revealed by local lobes with sharp chilled margins and internal flow banding that are compositionally different from the host lapilli tuff breccia or lapilli tuff in eastern and central seg-ments (Fig. 7b, Fig. 9b, c). Example of geometri-cal and internal organization of flows is the 100 – 350 m-thick massive to lobate Qp1 flow (la-beled I in Figs. 5 and 6), which contain a basal lapilli tuff breccia (Fig. 13a) and opposing lobe closures that crops out in the central segment. Detailed mapping of this Qp1 flow (Fig. 5b, Fig. 6b) shows east-closing lobes in the Qp1 flow (Fig. 6b, Fig. 13b) and in an aphanitic sill (Fig. 6b, Fig. 13c) that occur in the east, whereas west-closing lobes with lapilli tuff breccia (Fig. 13d) are
found in the west part of the central segment (Fig. 5b).
3.3.1. Interpretation
Andesitic massive and pillowed flows of phases 2a and 2c represent, as in phase 1, calm effusive subaqueous volcanism. Lapilli tuffs, the lateral and up-section continuation of the flows, are hyaloclastites (Cas and Wright, 1987; McPhie et al., 1993).
Fine to coarse-tuff and lapilli tuff of phase 2b (Fig. 8c) were deposited by high- to low-concen-tration turbidity currents, and are referred to as turbiditic tuffs (Mueller and White, 1992). Mas-sive to graded portions of beds are Bouma (1962)
Ta or S3-beds of Lowe (1982) deposited from
massive fallout during transport of high-concen-tration turbidity flows. The laminated and rippled
portions of the tuff represent Bouma Tbcparts of
beds. The matrix-supported lapilli tuff breccia
Fig. 11. Outcrop map of two representative andesitic flows of phase 2c from the eastern part of the central segment crosscut by Qfp3 rhyodacitic dyke of phase 4 (see Fig. 6 for location).
(Fig. 8e) is interpreted as a cohesive debris flow deposit (Lowe, 1982) and clast-supported lapilli tuff breccia of the western (Fig. 8f) and central (Fig. 9a) segments is best explained as a mass flow product transported downslope via laminar or plug flow (Lowe, 1982; McPhie et al., 1993). These fragmental deposits may be either pyroclas-tic or autoclaspyroclas-tic in origin (Cas, 1992) that were subsequently redistributed down-slope via sedi-ment gravity flow processes. Resedisedi-mentation of pyroclastic deposits is possible, but remobilization of autoclastic debris or hydroclastic products derived from subaqueous lava flow-water interac-tions (Fisher and Schmincke, 1984) appears more probable. Autoclastic or hydroclastic origin is
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columnar jointing to lobes with massive interiors and flow-banded margins, which in turn pass into viscous flow breccias and hyaloclastites (Figs. 4 and 5), are strikingly similar to the facies models of de Rosen-Spence et al. (1980) and Yamagishi and Dimroth (1985) for Miocene and Archean analogues. The lobes may be extrusive (exogenic; Figs. 4, 5 and 13d), or endogenic and intrude a pre-existing breccia pile (Figs. 5, 7b, 9b and 13c). Larger km-scale domal structures with intrusive
contacts and a massive appearance are consistent with endogenic domes (McPhie et al., 1993).
3.4. Phases 3 and 4:rhyodacitic-rhyolitic subaqueous 6olcanism
The 600 m-thick, aphanitic rhyodacite-rhyolite of phase 3, present in the western and central segments (Figs. 4 and 5; Table 1), display two laterally continuous 10 km long and 75 – 275
Fig. 13. Characteristics of phase 2c Qp1 rhyolite in the eastern and western parts of the central segment (locations for photos A, B, C are shown on Fig. 6b and photo D to Fig. 5b). Large white arrow indicates younging direction to the south in B – D. Pencil (15 cm) and hammer (35 cm) for scale. (A) Clast-supported Qp1 lapilli tuff breccia with subangular fragments with arrow showing 80 cm-size clast. (B) Meter-scale east-closing lobe with flow banding. (C) East-closing endogenic lobe in aphanitic sill intruded in Qp1 lapilli tuff breccia. (D) Meter-scale west-closing exogenic lobe with flow banding. Pencil displays massive part of lobe and black arrow shows brecciated portion.
thick flow units, as well as decametre-scale sills and dykes. Flow units display a change over 2 – 3 km, from massive to 3 – 30 m-thick massive or flow-banded lobes to lapilli tuff breccia, which grade up-section and laterally into massive, 1 – 10 thick lapilli tuff (2 – 5 csize) and 1 – 10 m-thick laminated tuff composed of 2 – 10 cm-m-thick beds (Fig. 5a). Extensively developed 8 – 15 cm-wide columnar joints are prominent in the basal parts of flows (Figs. 4a and 14a). Sills and dykes crosscut the previous phase 2 flows and feature
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The 475 m-thick rhyodacite-rhyolite of phase 4, constrained to the central segment, is composed of a massive unit and dykes (Figs. 2 and 6) with exclusively large quartz and feldspar phenocrysts (Qfp3 in Table 2; Fig. 16a). The 15 – 20 m-thick Qfp3 dykes cut phase 2 rocks (Fig. 6b, Fig. 11, Fig. 16b, c). The dykes are massive with 30 cm-thick flow banded margins (Fig. 16b) and 10 – 15 cm-thick chilled margins at the contact with Qp1 lapilli tuffs of phase 2c (Fig. 16c).
3.4.1. Interpretation
The felsic massive, lobate, and hyaloclastite flows of phase 3 in the central-west segment (Fig. 5) show the classical subaqueous morphological
facies organization of subaqueous flows (de Rosen-Spence et al., 1980). The fragmental units are autoclastic products and together with lobes represent the lobe-hyaloclastite flows of Gibson et al. (1997). The ambient aqueous environment fa-cilitated the brecciation process and the formation of columnar jointing in massive facies of flows. Associated lapilli tuffs and laminated tuffs, trans-ported via high- and low-concentration turbidity flows (Lowe, 1982), are either reworked hyalo-clastite breccia or local explosive hydroclastic products transported down-slope (Fisher and Schmincke, 1984). The geometry, large-scale change in flow band orientation from NW-SE to NE-SW and intrusive nature of the unit in the
Fig. 15. Outcrop map showing phase 3 rhyolite of the central segment of the NVC (see Fig. 2 for location). (A) Massive to flow-banded flows with associated breccia form meter to decameter horizons. (B) Position of outcrop in A with respect to an endogenous dome (modified from Burt and Sheridan, 1987).
central segment favor an endogenous dome struc-ture (Burt and Sheridan, 1987; Manley, 1996). Flow-banded fragments with a jigsaw-fit texture in a low intraclast matrix is typical of in-situ brecciation and common to dome margins (Allen,
1992; McPhie et al., 1993).
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feeders to the dome. The 10 – 15 cm-thick chilled margins of the dykes attest to a significant speed of cooling whereas flow banding is results of viscous magmatic flow and internal friction (Kano et al., 1991).
3.5. Sedimentation: 6olcanic quiescence
The Norme´tal sedimentary rocks represent a 20 – 70 m-thick volcaniclastic sedimentary marker horizon, which can be traced for ca. 35 km. The
Fig. 17. Characteristics of Norme´tal sedimentary rocks and phase 5 lapilli tuff of the NVC (located in Fig. 5). (A) Intrafolial folds in the volcanic siltstone-mudstone facies near Norme´tal. Measuring tape, 80-cm. (B) Turbiditic lapilli tuffs associated with the Normetmar massive sulphide deposit. Strong stretching of the lapilli (white arrow) defines down-dip lineation related to the Norme´tal deformation zone. Pencil, 15 cm.
3.5.1. Interpretation
Sharp contacts and normal grading of the vol-canic breccia-sandstone beds are consistent with
the S3 division of Lowe (1982). The
thin-lami-nated nature of the siltstone and mudstone
repre-sents Tde divisions (Bouma, 1962). The
predominance of mudstone suggests suspension-derived background sedimentation. These charac-teristics coupled with the absence of wave-induced sedimentary structures favors deposition in water depth in excess of 200 m.
3.6. Phase 5: mine sequence
Most of the information concerning the 100 – 400 m-thick Mine Sequence comes from diamond drill observations. Massive mafic and felsic flows or intrusions as well as abundant lapilli tuff brec-cias, lapilli tuff and fine tuffs have been identified. A highly altered quartz-sericite-carbonate-chlori-toid felsic tuff traceable for 30 km along strike (referred here as the Mine horizon) is associated with the VMS deposits. The most representative outcrop of the Mine sequence consists of graded-bedded tuff and lapilli tuffs and lapilli tuff brec-cias exposed at the Normetmar deposit (Figs. 5 and 17b). The volcaniclastic unit contains 2 – 5 m-thick fining-upward sequences in which grain size and bed thickness generally decrease up-sec-tion. The tuff, with parallel, undulating and rip-pled laminations, is composed of lithic felsic aphanitic and feldspar-phyric clasts.
3.6.1. Interpretation
Massive mafic to felsic flows are products of effusive volcanism. Tuffs, lapilli tuffs and lapilli tuff breccias are deposited by high and low-den-sity gravity flows (Bouma, 1962; Lowe, 1982) and may either be the reworked counterparts of auto-clastic breccia, a slumped carapace breccia or represent deposition from hydroclastic eruptions (Fisher and Schmincke, 1984).
3.7. Geometry and edifice construction
Lateral variations of volcanic facies in mafic-felsic rocks and lobe closures in viscous rhyolites delineate flow direction and infer vent location volcaniclastic rocks are composed of graded
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(Figs. 4 – 6). The flow direction coupled with dis-tinctive stratigraphy enables the recognition of three probable emission centers, which are located in the western, central and eastern segments (Fig. 18). Synvolcanic faults that acted as lava conduits and conduits for hydrothermal fluid circulation, are interpreted from detailed mapping (Fig. 10) and (1) changes in flow directions; (2) volcanic facies changes across Proterozoic dykes; (3) pres-ence of Proterozoic dykes; (4) thickness variations and distribution of units; and (5) disruption of geophysical conductors (Tessier, 1991).
The geometry of the NVC is interpreted as a multivent edifice (Fig. 18). The western vent is marked by opposing flow directions on either side of a synvolcanic fault (Figs. 4 and 18). With time, effusion of flows decreased whereas flows emanat-ing from the central vent covered the western
centre (Figs. 4 and 18). Numerous endogenous domes inflated the western volcanic sequence to mark final volcanic activity (Fig. 18). The central vent shows opposing flow directions and closure of the Qp1 flow (labeled I in Figs. 5 and 6), indicating the location of a major eruptive center probably about 2 km east of the Norme´tal deposit (Figs. 5, 6 and 18). Numerous synvolcanic faults and endogenous domes, dykes and sills of phases 3 and 4 as well as phase 3 flows also characterized the central vent. The eastern vent area is charac-terized by an aphanitic flow unit for which the emission centre is poorly defined. Turbiditic sedi-mentary rocks and pelagic mudstone of the Norme´tal sedimentary unit caps the volcanic se-quence with the exception of the phase 4 dome of the central vent, which represent probable resur-gent dome. Renewed volcanism of phase 5
ered the entire NVC. These characteristics argue for the development of a central cauldron struc-ture with minor vents to the east and west on an andesitic edifice (phase 1).
4. Geochemistry
Geochemical analyses were conducted to: (1) classify the rocks (e.g. Winchester and Floyd, 1977); (2) confirm contacts between volcanic phases; (3) distinguish between tholeiitic, transi-tional or calc-alkaline magma groups (Lesher et al., 1986; Barrie et al., 1993; Barrett and MacLean, 1997); and (4) determine whether sub-duction-related processes occurred. A total of 109 samples were analyzed for major and some trace (Zr, Y, Cu, Zn, Ni, V) elements using ICP-AES at the TSL Assayers Laboratories (Mississauga, On-tario). All major elements have 0.01% detection
limit except K2O and P2O5 (0.02%). Zr (10 ppm
detection limit), Y (2 ppm detection limit) and Cu, Zn, Ni, V (5 ppm detection limit) have detec-tion limits similar to standard ICP-AES analysis (Jenner, 1996). A total of 77 samples were chosen to determine a wider range of trace (As, Ba, Co, Cr, Cs, Hf, Rb, Sb, Sc, Ta, Th, U and W) and rare earth elements (REE) by instrumental neu-tron activation analysis (INAA) conducted at the Universite´ du Que´bec a` Chicoutimi (UQAC) us-ing the method of Bedard and Barnes (1990). In order to monitor the precision a standard was used in every run. Results for this sample have shown that the standard error is less than 5% for As, Co, Cr, Rb, Sc, Th, La, Ce, Yb, Lu and less than 10% for the other elements (Bedard and Barnes, 1990). A total of 20 representative sam-ples of the different volcanic phases are provided in Table 3.
4.1. Major element geochemistry
Volcanic rocks of the NVC display two com-plete compositional spectra from basaltic andesite to rhyolite (Fig. 19). Phase 1 of the NVC is marked by basaltic andesite to dacite, whereas phase 2 displays a trend from andesite to rhyolite. NVC phases 3 and 4 display rhyodacitic to
rhy-olitic compositions, and phase 5 exhibits a re-newed evolution from basaltic andesite to rhyolite suggesting a cyclic evolution of the magmatic system.
4.2. Zr/Y ratios
The geochemical affinity of the volcanic rocks in the NVC can be determined by using the incompatible Zr-Y trace element discrimination diagram of Barrett and MacLean (1997) which is commonly employed to characterized association of VMS deposits with Archean volcanic rocks (Lesher et al., 1986; Barrie et al., 1993). The Zr/Y ratios indicate that basaltic andesites of phase 1, felsic rocks of phase 2 as well as rhyodacites and rhyolites of phase 4 are of transitional affinity (Fig. 20). The andesite-dacite of phase 2 with an
average Zr/Y ratio of 7.5 straddles the boundary
between transitional and calc-alkaline. Despite the wide range in composition, volcanic rocks of phase 5 display a well-defined linear array, similar to combined phases 1 and 2, with an average of
Zr/Y=6.7 and classify as transitional (Fig. 20).
In contrast, rhyodacites and rhyolites of phase 3
with a very low Zr/Y average of 2.0 define a
distinct tholeiitic trend (Fig. 20).
4.3. Rare-earth element (REE) geochemistry
The LaN/YbN ranges of Barrett and MacLean
(1997) are used to characterize rock affinity: (1) LaN/YbN=1 – 3, tholeiitic; (2) LaN/YbN=3 – 6,
transitional; and (3) LaN/YbN=\6,
calc-alka-line. Andesites and dacites of phases 1 and 2 show a slightly fractionated LREE to HREE pattern, whereas rhyolites of phase 2 show an enrichment of LREE and a near-flat distribution of HREE
(Fig. 21). Based on average LaN/YbN, phases 1
and 2 are classified as transitional (Table 3; Fig. 21). The slightly calc-alkaline nature of phase 4
(average LaN/YbN=6.33) is consistent with its
classification as the more evolved member of the first volcanic episode. Mafic and felsic flows of phase 5 exhibits REE patterns similar to phases 1
and 2. The REE patterns, as well as Zr/Y ratios
B
.
Lafrance
et
al
.
/
Precambrian
Research
101
(2000)
277
–
311
299
Table 3
Major and trace-element composition of representative analyses from the Norme´tal volcanic complexa
aNotes: bas. and, basaltic andesite; apha rhy, aphanitic rhyolite; Qfp2, quartz-feldspar phenocrysts with B5% qtz, 0.5–1 mm;
B.Lafrance et al./Precambrian Research101 (2000) 277 – 311 301
B.Lafrance et al./Precambrian Research101 (2000) 277 – 311 303
arc construction (Barrett and MacLean, 1997). Felsic rocks of phase 3, distinct from the other phases, straddle the tholeiitic to transitional
boundary (average LaN/YbN=3.02) and have a
more pronounced negative Eu anomaly than phases 2 and 4. These features suggest a more primitive source where subduction-related meta-somatism is either not-or less involved (Wilson, 1989; Kerrich and Wyman, 1997). Phase 3 is possibly related to a magmatic source associated with initiation of arc rifting.
5. Evolution of the NVC
The physical volcanology of various segments enables paloegeographic reconstruction of the NVC and the geochemistry helps place the NVC in a geodynamic context. Our model for the evo-lution of the NVC in different stages is shown in Fig. 22. The first stage (phases 1 and 2a) is characterized by transitional effusive volcanism of basaltic andesite, andesite and dacite, representing construction of a shield volcano on a basaltic lava plain (Fig. 22a). A shift from an initial shield-type edifice to a composite volcano with numerous emission centers is recorded during phase 2b (Fig. 22b). These hydroclastic or autoclastic-derived volcaniclastic deposits form a laterally limited stratigraphic level marking the isolated vents. A central cauldron structure formed by progressive subsidence along synvolcanic faults during this phase. Phase 2c volcanism (Fig. 22c) characterized by massive, lobate and brecciated rhyolite with interstratified dacite and pillowed or massive an-desite indicates major effusive volcanism in the central vent and in parasitic western and eastern vents. Large volumes of lava erupted in the cen-tral vent subsequently covering the minor eastern and western individual vents, permitting coales-cence of individual centers. All the previous units are geochemically characterized by transitional affinity. The tholeiitic to slightly transitional phase 3 that include massive, lobate and brec-ciated rhyolite flows as well as the endogenous dome ascended through other synvolcanic faults that access a different magmatic chamber in the central vent (Fig. 22d). Synchronous with phase 3
activity, phase 4 volcanism of transitional affinity caused endogenous dome formation and dike in-trusion in the central cauldron (Fig. 22d) and phase 2 endogenous lavas developed in the west-ern vent. A period of volcanic quiescence with below wave base background sedimentation and deep-water volcaniclastic turbidites indicate that the NVC was constructed in a subaqueous setting (Fig. 22e). Hydroclastic volcanism and minor ef-fusion characterize renewed transitional phase 5 volcanism (Fig. 22f). The hydrothermal system responsible for the massive sulphide deposits de-veloped on the western margin of the central cauldron structure along a synvolcanic fault (Figs. 18 and 22f). Similar models with VMS deposits forming at a cauldron margin have been made by Gibson and Watkinson (1990) for the Noranda cauldron and the active VMS-Sunrise deposit (Iizasa et al., 1999). All these characteristics are compatible with the development of an arc-related volcanic complex.
6. Discussion
6.1. Modern arc-related6olcanoes
Fig. 22. Evolution for the Archean Norme´tal volcanic complex with individual phases explained in text. Note the change from a mafic-dominated shield volcano (A) to a felsic-dominated composite volcano with three principal vent sites (B – D). Volcanic quiescence is indicated in E and renewed volcanic activity and VMS formation is depicted in F.
cauldron zone are consistent with composite vol-canoes. High temperature of emission, elevated heat retention capability, high magma discharge rates and low magmatic water content as well as deep-water setting could explain the dominant effusive style of the NVC felsic rocks (Cas and
Wright, 1987; Dadd, 1992; Manley, 1992, 1996). Despite the fact that Mt. Etna is a subaerial volcano, its morphological evolution from a basal
38×47 km shield volcano with effusive and
B.Lafrance et al./Precambrian Research101 (2000) 277 – 311 305
central 6 km-wide emission centre is consistent with the model for the NVC. The edifice construc-tion of subaerial stratovolcanoes, such as Nevado de Colima, Mexico and Mt. Etna, Italy that com-monly display three principal stages: (1) initial basaltic-andesitic effusive volcanism; (2) felsic ex-plosive and effusive volcanism; and (3) a late-stage dome building phase (Kieffer, 1995; Robin, 1995) is also consistent with the NVC evolution. Submarine analogues for the NVC include the mafic-felsic centers of the Sumisu rift zone (Smith et al., 1990) and the Myojin Knoll caldera (Iizasa et al., 1999) both of the Izu-Bonin arc, and the Rumble volcanoes associated with a cross-arc rift zone in the southern Havre trough (Wright et al., 1996, 1998).
The evolution of the NVC must be considered within the context of the Abitibi greenstone belt that has been described as a collage of island arc segments (Mueller et al., 1996). The northern volcanic zone of the Abitibi belt is interpreted as a diffuse arc segment with early linear intra-arc sedimentary basins (Chown et al., 1992; Mueller and Donaldson, 1992). The location of the NVC within the arc remains problematic but, as docu-mented by Smith et al. (1990) for the Izu-Bonin arc, complex volcanic centers with abundant hy-drothermal activity may form in the transition zone from the arc to backarc. A similar setting for the NVC is proposed.
6.2. Geochemical signature of arc setting
The geochemical signature of the arc geody-namic setting is rendered complex by the interplay of varying degrees of subducted slab dehydration, partial melting of the mantle wedge or the sub-ducted slab, and crustal contamination. Presence of multisource and multistage magma generation
in this environment is generally recognized
(Wilson, 1989). Moreover, rifting of the arc and ultimately the creation of mature backarc or in-tra-arc basins (with volcanic spreading) favor the shifting from arc signature to MORB signature with several intermediate geochemical signatures possible (Hochstaedter et al., 1990; Sato and Amano, 1991; Barrett and MacLean, 1997).
Multi-element diagrams (spidergrams) charac-terize magmatic affinity and suggest geodynamic setting (Sun, 1980; Sun and McDonought, 1989; Kerrich and Wyman, 1997). Phases 1 and 4 (Fig. 23a) show Ta depletions relative to Th and La as well as Ti depletion which commonly suggests subduction-related petrogenesis (Wilson, 1989; Kelemen et al., 1993; Brenan et al., 1994) al-though crustal contamination cannot be ruled out (Wilson, 1989; Kerrich and Wyman, 1997).
Re-cent geochemical and oNd isotope studies of the
Hunter Mine Group (Fig. 1; Dostal and Mueller, 1996, 1997), as well as selected plutonic suites in the Abitibi greenstone belt (Bedard and Ludden, 1997), show that no significantly older crust was involved with the genesis of the plutonic and volcanic rocks of the Abitibi belt. The multi-ele-ment diagrams therefore support an arc setting, rather than crustal contamination. A low normal-ized abundance of Ta can be as a result of either retention of insoluble mineral phases such as ru-tile, titanite or perovskite in the subducted slab (Brenan et al., 1994) or the retention of orthopy-roxene, garnet, spinel or olivine in the mantle wedge during reaction between migrating melts and the upper mantle (Kelemen et al., 1993).
6.3. Geochemical comparison of Abitibi 6olcanic centers
Felsic volcanic rocks in the Abitibi belt have been classified using trace elements to determine the geochemical affinities of volcanic rocks associ-ated with VMS deposits. Lesher et al. (1986) and Barrie et al. (1993) have documented a spatial and likely genetic association between tholeiitic or
transitional host rocks and VMS deposits,
whereas most calc-alkaline sequences are not fa-vorable for VMS formation. Phase 2, 4 and 5 rhyolites of the NVC best classify as transitional group II and phase 3 rhyolites as tholeiitic group I of Barrie et al. (1993).
B.Lafrance et al./Precambrian Research101 (2000) 277 – 311 307
tholeiitic nature of the Matagami rhyodacite (Piche´, 1991), Waconichi rhyolite of Chibouga-mau (Ludden et al., 1984), and Joutel rhyolite (Dube´, 1993), the transitional trend for the NVC, and a distinct calc-alkaline affinity for the Hunter Mine Group (Dostal and Mueller, 1996). This diagram also illustrates the decreasing content of REE, particularly HREE, from north (Matagami, Chibougamau, and Joutel) to south (Norme´tal, Hunter Mine).
Spidergrams discriminate between sequences with Ta depletion (Fig. 23c; Norme´tal phase 2, Hunter mine rhyolite) and sequences without Ta depletion and stronger Ti negative anomalies (Fig. 23d; Matagami rhyodacite, Joutel rhyolite and phase 3 of Norme´tal). These differences are con-sistent with an arc setting that contains a
transi-tional to calc-alkaline source derived from
subduction-related processes (Norme´tal phase 2, Hunter mine rhyolite) and a more primitive tholeiitic source derived from less involved sub-duction processes (Matagami, lower cycle of Chi-bougamau, Joutel rhyolite and phase 3 of Norme´tal). This is consistent with the rift-related setting proposed by Barrett and MacLean (1997) for the Matagami sequence. The record of arc-re-lated and rift-rearc-re-lated volcanism in the NVC
sug-gests that Norme´tal represents a transition
between a frontal arc setting (Hunter Mine, south) and a more mature arc-related rift zone (north).
7. Conclusions
Regional mapping and detailed volcanological facies permitted the identification of three individ-ual volcanic emission centers that coalesced dur-ing the evolution of the complex. The largest central cauldron features a 6-km wide structure, whereas the western and eastern vents are subor-dinate in size. Criteria for the recognition of the felsic centers were proximal-distal morphological flow variations and lobe closures. Individual felsic flow units defined by the phenocryst content, size and proportion could be followed intermittently for kilometres along strike. Massive, pillowed and brecciated andesite flows and viscous felsic
mas-sive to lobe-hyaloclastite flow characterize this effusive dominated volcanic complex. Volcaniclas-tic deposits generally originate from reworking of autoclastic flow breccias. Pelagic background sed-iments and turbiditic tuffs attest to a subaqueous setting well below storm wave for the NVC. The NVC is interpreted as a multivent, mafic-felsic, effusive-dominated composite volcano that devel-ops on a basal shield volcano.
The mafic-felsic NVC of the Abitibi greenstone belt has a dominant transitional and subordinate tholeiitic character as indicated by Zr/Y and LaN/
YbN ratios. The trace element variations of the
various phases are consistent with a multisource subduction-related island arc setting where sub-duction processes are involved to varying degrees. The presence of these two distinct geochemical affinities suggests that the NVC represents volcan-ism from nascent or immature arc of transitional geochemical affinity and a more mature
arc-re-lated rift (tholeiitic to slightly transitional
affinity).
The diagnostic features for VMS deposits loca-tion in the NVC are (1) localoca-tion centered on major divergent lava flow directions; (2) location over synvolcanic faults at the western margin of the central cauldron zone; and (3) spatial associa-tion with phase 5 turbiditic lapilli tuff breccia, lapilli tuff and tuff which shows transitional affinity and strong alteration. The NVC is consid-ered as an ancient counterpart of modern sub-aqueous arc edifice like these of the Izu-Bonin and Kermadec arcs that display hydrothermal activity and VMS development along the arc or are slight removed towards the backarc.
Acknowledgements
S. Desbien and R.H. Dufresne are thanked as well as P. Be´dard for REE determination (INAA). Journal reviewers J. Ayer and P.C. Thurston are gratefully acknowledged for their incisive criti-cisms and editing.
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