arc construction Barrett and MacLean, 1997. Felsic rocks of phase 3, distinct from the other
phases, straddle the tholeiitic to transitional boundary average La
N
Yb
N
= 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-related 6olcanoes Modern arc volcanoes display a diversity of
volcanic landforms ranging from steep-sided stra- tovolcanoes or composite volcanoes of rhyolitic
and andesitic composition such as Tongariro and Ruapehu of the Taupo volcanic zone Wilson et
al., 1995, Epi Island, Vanuatu MacFarlane et al., 1988, and Etna, Italy Kieffer, 1995, to
low-angle shield volcanoes such as Ambrym Robin et al., 1993 and Aoba, Vanuatu MacFar-
lane et al., 1988. Large ash flow calderas such as Mangankino and Taupo Wilson et al., 1995 and
Kuwae, Vanuatu, Monzier et al., 1994 are also inherent to arc settings. The absence of large
volume ignimbrites typical of ash flow caldera formation rules out this type of volcanic construc-
tion for the NVC. The arc-related NVC is best explained as a submarine composite volcano de-
veloped upon a shield volcano. The NVC is simi- lar to shield volcanoes in terms of dominant
effusive volcanism, whereas the andesitic base, a large volume of felsic rocks and a large central
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 ex-
plosive mafic volcanism to a complex of felsic pyroclastic and effusive stratovolcanoes within a
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.
Compiled REE and trace element data of six felsic or mafic-felsic complexes located on Fig. 1
associated with economic or non-economic VMS deposits Hunter Mine group are consistent with
multisource and multistage magma generation in an arc setting. REE patterns Fig. 23b display the
Fig. 23. A Primitive-mantle normalized spidergrams Sun and McDonought, 1989 using immobile elements for the volcanic rocks of the phases 1 and 4 of the NVC. B Chondrite normalized REE diagrams Nakamura, 1974 for six different volcanic complexes
of the Abitibi. C Primitive-mantle normalized spidergrams Sun and McDonought, 1989 using immobile elements for the volcanic rocks of the Norme´tal phase 2 and Hunter Mine rhyolitic dyke, which both show Ta depletion. D Primitive-mantle normalized
spidergrams Sun and McDonought, 1989 using immobile elements for the volcanic rocks of the Matagami rhyodacite, Joutel rhyolite and phase 3 Norme´tal rhyodacite-rhyolite. Note the absence of Ta depletion and the stronger Ti negative anomalies.
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-related 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
