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 K
2
O and P
2
O
5
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
. ZrY 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 ZrY
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 ZrY 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 ZrY = 6.7 and classify as transitional Fig. 20.
In contrast, rhyodacites and rhyolites of phase 3 with a very low ZrY average of 2.0 define a
distinct tholeiitic trend Fig. 20.
4
.
3
. Rare-earth element REE
geochemistry The La
N
Yb
N
ranges of Barrett and MacLean 1997 are used to characterize rock affinity: 1
La
N
Yb
N
= 1 – 3, tholeiitic; 2 La
N
Yb
N
= 3 – 6,
transitional; and 3 La
N
Yb
N
= \ 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 La
N
Yb
N
, phases 1 and 2 are classified as transitional Table 3; Fig.
21. The slightly calc-alkaline nature of phase 4 average La
N
Yb
N
= 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 ZrY ratios
of phases 1, 2, 4 and 5 are consistent with transi- tional volcanism related to nascent or immature
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 complex
a
a
Notes: bas. and, basaltic andesite; apha rhy, aphanitic rhyolite; Qfp2, quartz-feldspar phenocrysts with B5 qtz, 0.5–1 mm; Qfp2, quartz-feldspar phenocrysts with 10–25 qtz, 1–2 mm; Qp1, quartz phenocrysts with 10–25 qtz, 1–2 mm; Qfp3,
quartz-feldspar phenocrysts with B5 qtz, 0.3 mm–1 cm. Major elements and Zr, Y, Cu, Zn, Ni, V from ICP-AES. Other elements from INAA.
Fig. 19. Geochemical classification of volcanic phases for the Norme´tal volcanic complex. Compositional field boundaries according to Winchester and Floyd 1977. Phases 1 – 4 display a compositional spectrum from basaltic andesite to rhyolite that is referred to
as the first volcanic phase. The second volcanic phase is composed of phase 5 basaltic andesite to rhyolite.
Fig. 20. Y versus Zr diagrams for the different volcanic phases of the Norme´tal volcanic complex. Field boundaries after Barrett and MacLean 1997.
Fig. 21. Chondrite-normalized Nakamura, 1974 REE diagrams for the different volcanic phases of the NVC.
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
