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
