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Petrogenesis and tectonic significance of amphibolites interlayered with metasedimentary gneisses in the Ivrea Zone, Southern Alps, northwest Italy
Tectonophysics,
PETROGENESIS INTERLAYERED
B.V.. Amsterdam
(Received
of Geolqy,
November
in The Netherlands
ALPS, NORTHWEST
D. SILLS and JOHN
Department
- Printed
AND TECTONIC SIGNIFICANCE OF AMPHIBOLITES WITH METASEDIMENTARY GNEISSES IN THE IVREA
ZONE, SOUTHERN
JANE
187
107 (1984) 187-206
Elsevier Science Publishers
ITALY
TARNEY
Uniuersity
of Leicester, Lercester. LEI 7RH (United Kingdom)
7, 1983; revised version accepted
February
27. 1984)
ABSTRACT
Sills, J.D. and Tarney, metasedimentary
J., 1984. Petrogenesis gneisses
and tectonic
in the Ivrea Zone,
significance
Southern
of amphibolites
Alps, Northwest
Italy.
interlayered
with
Tectono&sics,
107:
187-206. Amphibolites formation
occurring
of the lvrea
characteristics
as elongate
Zone
(N-type)
have higher basalts.
and
mid-ocean
distributions
ridge basalts.
metamorphism
showing
appears
elements
basaltic
The tectonic formed
geochemical
deformed
at a (Palaeozoic)
study
of their
to transitional the rare-earth
elements
typical
layers and boudins
or enriched
in loss of SiO, and variable
including
element
occur as layers up to
in incompatible
occur as thinner
(kinzigite) trace
(E-type)
enrichment
elements,
of and
ocean in Na.
have preserved
characteristics.
model which best explains
highly
the metasedimentary
Type 1 amphibolites
depletion
similar
to have resulted
K. Rb, Ba, Sr and locally Ca, the less mobile elements,
extremely
within
A detailed
Type 2 amphibolites
levels of the more incompatible
Although
the primary
boudins
parentage.
shows that they can be divided into two groups.
100 m thick and have trace-element normal
lenses
are all of igneous
semi-pelitic
subduction
the assemblage
metasediments
of slivers of MORB-like is that
of an accretionary
amphibolites
within
sedimentary
wedge
zone.
INTRODUCTION
The occurrence of amphibolite layers within metasedimentary sequences poses problems as to their origin, particularly where the rocks are so deformed that any original intrusive or sedimentary relationships have been totally destroyed. Such is the case with amphibolites occurring within the metasedimentary gneisses of the Ivrea Zone of the Southern Alps in northwest Italy (Mehnart, 1975). This region is of special interest because it is commonly regarded as a section of the deeper crust (Mehnart, 1975; Fountain and Salisbury, 1981) and therefore can provide information on the processes of crustal development. Amphibolites may be derived from basaltic rocks, from calcareous or dolomitic
0040-1951/84/$03.00
0 1984 Elsevier Science Publishers
B.V.
lxx
shales (Leake, and
carbonate
impossible
1964). or develop bands
to produce
to determine
thin
their origin
must be made to geochemical is unlikely
through
layers
exchange
(Orville,
Although
and significant
occurred, Leake (1964) has shown that primary cal trends may be preserved, thus permitting para-amphibolites.
between
adjacent
1969). In many
from field or textural
techniques.
to have been isochemical
chemical
observations.
metamorphism compositional
pelite
cases it is so recourse
of amphibolites change
may have
magmatic or sedimentary geochemidiscrimination between ortho- and
If the amphibolites have a basaltic protolith. it is vital to assess the extent of low-temperature alteration which is known to have a major effect on the composition (e.g.. Hart, 1970; Cann, 1969). However, several studies have shown that some high field-strength elements (Ti. Zr, Nb, Y) remain relatively immobile during metamorphism and may be used to indicate the original basalt type and possible tectonic setting (Pearce and Cann. 1973: Floyd and Winchester, 1975: Stephenson and Hensel, 1982). This study investigates the geochemistry of the amphibolites and considers their origin and likely tectonic setting which may bear upon the evolution of the Ivrea Zone as a whole. GEOLOGICAL
SE’MING
The Ivrea Zone (Fig.
1) consists
of a steeply
dipping
series of metabasites
and
metasedimentary gneisses of Palaeozoic age inferred by many authors to be a deformed section through the lower continental crust (e.g. Mehnart. 1975). The metasediments (termed “kinzigite” at amphibolite facies and “stronalite” at granu-
Fig. 1. A. Sketch map of the Ivrea Zone. 1980.) Crosses amphibolites: layered
B. Sketch map of Val Strona and Val d’ossola. (After Zingg. IV28. 29, 30, 200, 201. 386. 390, 391 and 391A are type 1 IV21 and 193 are related to the IV25. 26, 32, 199. 378. 379 and 382 are type 2 amphibolites: mark sample localities.
complex.
Samples
189
lite facies) are a heterogeneous Metamorphosed
mafic
mixture
rocks make
of pelite, semi-pelite
up a significant
and carbonate
proportion
rocks.
of the Ivrea Zone
(Fig. 1) and are of two kinds: (a) a large
layered
mafic-ultramafic
Finer0 (Rivalenti et al., 1975) and (b) layers of amphibolite within
complex,
the metasediments.
occurring
in Val Sesia and
It is the amphibolites
at
which
form the subject of this paper, representatives of which are best exposed in Val Strona (Fig. 1). The metamorphic grade increases from amphibolite facies in the southeast to granulite facies in the northwest (Mehnart, 1975; Schmid and Wood, 1976; Hunziker and Zingg, 1980) and the prograde petrological changes in the metabasites have been described in detail by Reinsch (1973a,b); Mehnart (1975) and Zingg (1980). The mafic rocks investigated in this study are of three types: (a) Units of amphibolite, up to 100 m thick but commonly lo-20 m thick, interbanded with garnet-biotite gneiss occurring between Forno and Sambughetto in Val Strona and at Nibbio in Val d’Ossola (Fig. 1). These will be referred to as type 1 amphibolites. (b) Thin layers or lenses, 25-100 cm thick, complexly interfolded with the sediments. These are common in the amphibolite facies zones but also occur in the granulite
facies zones. These wiI1 be referred
to as type 2 amphibolites.
(c) Garnetiferous metabasites occurring at the base of the section at Campello Monti. These are continuous with the pyroxenites and metagabbros of Val Sesia (Capedri et al., 1977) but have a quite different composition from the other metabasites in Val Strona (Dostal and Capedri, 1979). The assumption that all the metabasites in the Ivrea Zone have a common origin (Mehnart, 1975: Zingg, 1980) is not necessarily correct. This paper concentrates on the bulk rock major and trace element chemistry of these
different
metabasic
rocks,
petrography, mineral chemistry elsewhere (e.g., Zingg, 1980).
and
is prefaced
and metamorphism,
by only
limited
discussion
of
as these have been described
In marked contrast to the gabbros from the layered complex, the amphibolites are medium- to fine-grained, well foliated rocks containing alternating felsic and mafic bands.
The amphibolites
in the lower part of Val Strona
comprise
50-70s
green
prismatic hornblende, 20-40% polygonal plagioclase, up to 10% biotite with accessory quartz, apatite and sphene (which usually rims ilmenite). As described by Reinsch (1973a), the colour of the hornblende changes systematically from green to brown as the metamorphic grade increases. The type 1 amphibolites contain up to 30 vol% of a pale green to colourless anhedral clinopyroxene. This has very low TiO,, AI,O,, (< 1.5 wt.%) and Cr,O, contents and has the approximate composition Ca 48-soMg83-.t6Fe15-,6. The clinopyroxene-bearing rocks are banded with pale
190
cl~nopyroxene-plag~~lase
and
darker
hornblende-rich
layers.
Accessory
minerals
include seapolite, sphene. opaque oxides, apatite, green biotite and carbonate. At Nibbio there are 10 cm long pegmatoid lenses containing scapolite
(Me,, ). plagioclase.
(andradite
54%. grossular
Near
Forno
sphene
and
unusual
apricot-coloured
garnet
1 wt.% TiO, ).
33%. with about
in Val Strona.
an
traces of epidate,
a 1 m wide ultramafic
layer consisting
of brown
hornblende, orthopyroxene and olivine occurs. Mafic granulite layers have a polygonal eyuigranular texture consisting of plagioclase, orthopyroxene. clinopyroxene and brown
hornblende
granulites
with accessory
at Campello
Monti
red biotite.
opaque
are garnetiferous
oxides and apatite.
and
pyroxene and plagiociase up to 0.5--l cm across 1 cm across but up to 2.5 cm across.
much
and
coarser
euhedral
The mafic
grained,
garnets
with
generally
0.5
CHEMISTRY
AND PETROGENESIS
Analyses were performed using a Philips PW1400 X-ray fluorescence spectrometer at the University of Leicester following analytical techniques outlined by Tarney et al. (1979a), Marsh et al. (1980) and Weaver et al. (1983). Briefly, major elements were determined on glass discs fused with a lithium tetraborate flux. and trace elements on pressed powder pellets. Major elements, Ni, Zr. Y, Zn, Sr, Rb and Ga were analysed using a rhodiun~ target and Cr. Ti, V, Ba, La, Ce and Nd using a
TABLE Analyses
1
(ppm) of Whin Sill. an Open
amphibolites.
1X1.
University “ill house” standard -I_lV147 -.-~ INAA
Whin Sill
Elem.
run
wth
the Val Strona
IV26. IV28 and IV201 *
fCP(7)
ICP(2)
~.--_ll_____-La
meaa.
recom.
25.0
25.0
19.8
19.8
21.3
<‘c
58.0
57.6
47.5
42.3
44.7
Nd
il.6
72.3
31.1
‘7.3
24.7
Sm
7.3
7.2
4.5
4.7
4.4
ELI
2.26
2.19
4.1
3.4
3.7
Cd
6.X
7.0
4.6
i.8
4.4
‘l-h
1.12
1.0x
0.5.:
Tm Yh
0.45 1.52
0.42 2.4x
0.64
0.6X
(1.74
1.11
0.38
0.37
0.08
0.11
0.14
* The recommended
value is an average of repeated
and JCP (twice) IO show the comparability analyses
is about
5%. for La. 10% for Cr.
analyses.
> 25% for Gd and Tm. For the ICP method
the analytical
the chemical
introduces
separation
technique
probably
IV147 is a sample analysed
hy both INAA
of the two techniques. The analytical precision for the lNAA < 5% for Sm. ELI. Tb and Yb, about 15% for Lu and Nd and precision
further
is I-6% (Walsh et al., 1981) although
errors.
191
TABLE
2
Type 1 amphibolites. Components
Val Strona and Val d’Ossola
Rock samples: IV28
IV28
IV30
IV200
IV201
46.6
47.3
48.4
IV380
IV386
IV390
46.0
46.3
IV391
IV391A
(wt.%)
Chemical ana!vses
47.8 1.92
0.95
1.45
1.97
48.8 1.89
TiO’ SiO
47.1 1.32
Al z;,
17.3
17.0
16.1
16.2
14.7
14.6
Fe203
0.98 16.8
1.72
47.3 1.79
45.9 1.78
13.4
13.9
14.0
11.12
14.35
10.34
10.54
13.76
13.35
9.64
13.33
13.50
13.88
MnO
0.18
0.22
0.18
0.16
0.22
0.21
0.16
0.27
0.19
0.23
MgO
7.06
6.19
11.02
7.28
5.90
6.34
7.76
5.82
6.68
6.17
CaO
13.29
9.22
13.64
14.78
11.60
11.47
16.50
16.27
12.18
15.43
NazO
2.22
3.47
0.99
2.18
2.62
2.31
1.64
2.63
3.35
2.46
K2O
0.86
0.63
0.86
0.72
0.74
0.67
0.63
0.35
0.92
0.44
p205
0.08
0.17
0.06
0.11
0.17
0.17
0.10
0.15
0.15
0.19
100.53
100.97
100.74
100.72
100.08
99.81
100.21
100.24
99.96
100.48
0.69
0.93
1.25
0.52
0.36
0.41
0.74
1.43
0.75
0.79
Total LO1 %
58.8
49.3
70.6
60.8
49.1
51.7
64.4
49.6
52.7
50.0
51
Trace elements @pm) Ni
96
63
299
153
68
70
226
45
50
Cr
298
276
494
345
199
223
598
90
87
96
V
263
352
164
242
663
374
185
321
348
318
Zn
106
129
86
84
116
117
77
93
108
108
Ga
17
23
15
18
20
18
14
19
18
20
Rb
21
7
17
11
6
7
11
4
11
3
Sr
108
157
189
177
125
122
137
171
96
127
Y
30
55
23
30
44
43
25
42
41
48
Zr
72
98
65
102
118
115
59
107
106
118
Nb
3
6
3
4
6
3
2
3
3
3
Ba
57
39
105
48
38
60
46
34
40
38
-cl
3
5.3
4
3.8
3
4.0
4
Ce
8.5
20
7
9
14.9
9
7.7
11
10.8
11
Nd
9.3
17
5
10
14.4
9
6.0
9
8.3
9
Sm
3.4
La
EU Gd Tb
5
<3
1.3 nd
4.8
2.4
1.6
0.8
1.2
3.0
4.8
3.3
4.8
nd
3.3
0.8
1.25
Yb
3.26
4.7
2.0
3.0
LU
0.5
0.71
0.33
0.49
DY
CIP W norms (wt.%) QZ
0
0
0
0
0
0
0
0
0
0
OR
5.1
3.7
5.0
4.2
4.4
4.0
3.7
2.1
5.4
2.6
AB
13.7
27.4
8.3
11.0
22.1
19.6
3.7
7.7
15.9
8.6
AN
34.5
28.7
36.7
32.1
26.1
27.5
36.5
23.7
20.2
25.7
Kc,ck wlple\: 11’2X N F.
I)1
IV200
IV28
2.7
0.0
3.0
71.1
12.9
32.7
IV380
IV.201 0
I\‘?‘11
IV?L)IA
5.5
(> 7
36.3
32.5
40.x
iI
0
0
7.5
‘.I
I\.‘?Y(l
I V?Xh
(3.6
H\i
0
0
0
4.:
13.4
OI-
13.4
1x.7
IN?
0.x
4.5
9.7
1141
S.?
M-l-
2.0
2.6
I .9
7.1
7.4
I7
14
2.5
ILM’I
2.5
3.6
7.7
17
.J.h
I.‘)
i.4
3.4
Ap
0.2
0.4
0.3
0.3
0.4
0.2
(I.4
(I.4
(1
tungsten target. Additional rare earth data were obtained for four samples (IV21, IV26, IV28 and IV201) by instrumental neutron activation analysis at the Open University
using
the method
IV386 and IV391)
of Potts et al. (1981) and
by inductively
London
using
(IV147).
analysed
techniques
Analyses
of type 1 amphibolites
using
coupled
described both
lites as well as metagabbros
by
techniques,
plasma Walsh
for three samples
spectrometry et al. (1981).
(IV378.
at Kings The
gives very comparable
College,
same
results
sample
(Table
1).
are given in Table 2 and those of type 2 amphibo-
from C’ampello Monti
are given in Table
3.
Nature of purent rocks
Chemical analyses istry the amphibolites
(Tables 2 and 3) show that, from their major element resemble aikalineeolivine basaits or olivine tholeiites;
chemabout
c Sphene
OPX
CPX
Fig. 2. ACF diagram for Val Strona amphibolites.
Field of basalts taken from Orvllie (1969).
this and subsequent figures are as follows:
+ = type 1 amphibolites.
of the most incompatible
amphibolites
element-enriched
x
=
from Sambughetto
type 2 amphibolites. (IV32.
IV378.
Symbols on l = three
IV379).
193
Fig. 3. Niggli 100 mg-c-(al-a/k)
half the samples rarely
quartz
plot within
and c-100
being
nepheline
normative.
mg diagrams
normative,
after Leake (1964). Symbols
the remainder
but with a spread
towards
However, a broad similarity with basaltic compositions origin as any rock consisting dominantly of hornblende its origin, will have a similar Chemical
trends
hypersthene
An ACF plot (Fig. 2) shows that the majority
the field of basalt,
para-amphibolites
being
as in Fig. 2
provide (Leake,
major element
compositions.
does not prove an igneous with plagioclase, whatever
chemistry.
the best means 1964). Figure
Ca-rich
or
of samples
of discriminating
3 shows
that
between
the Niggli
c-mg
ortho-
and
and
100
mg-c-( al-alk ) trends for all the amphibolites broadly follow an igneous rather than a sedimentary trend but there is a spread towards Ca-rich compositions for the clinopyroxene-bearing
type 1 amphibolites,
which does not eliminate
origin for some of this group. The type 2 amphibolites
a sedimentary
follow the igneous
trend more
closely. Ni
Cr
A
10
,40
50
80
70
Fig. 4. Cr vs mg and Ni vs rng (100 Mg/(Mg+ pelites is from Leake (1964).
1 t
,40
50
Fe2+ )) for amphibolites.
80
70
Symbols
as in Fig. 2. Field of
81
123
310
12s
24
Ni
Cr
V
Zn
Ga
Trace elemenrs lppm)
50.1
0.71
LO1
mg
100.37
Total
19
93
277
271
100
59.0
0.45
100.45
0.11
0.57
p205
2.82
10.44
0.42
11.26
CaO
7.60
3.26
4.47
MgO
0.16
11.90
0.68
0.19
MnO
K,O
14.17
Fe@.?
16.9
1.40
48.7
IV376
_
Na,O
14.5
3.37
45.9
AI 1%
TiO,
SO,
Chemica! anu~vsbs (wt.4 )
IV32
Rock samples:
of type 2 amphibolites
3
Components
Analyses
TABLE
21
91
21
248
X8
210
87
56.3
0.88
99.64
0.40
0.90
252
1x4
81
53.6
0.88
100.01
0.45
1.22
4.01
9.00
9.35
4.27
6.20
0.15
10.82
16.0
2.36
49.8
lV379
5.23
0.13
10.18
16.7
2.48
50.0
IV378
9
105
144
1976
1076
79.0
2.68
100.04
0.10
0.20
0.80
7.82
23.98
0.17
14.36
5.9
1.11
45.8
IV382
together with two samples (IV21
15
79
253
684
354
70.6
0.45
17
96
206
509
237
64.7
0.34
101.87
0.22
0.13 100.46
0.74
2.51
13.94
9.06
0.14
11.10
13.8
1.86
47.5
IV199
.71.
85
263
427
76
63.6
0.97
100.59
0.26
0.77
1.13
11.48
7.74
0.17
11.22
17.1
1.95
47.8
IV25
of metagabbroa
0.59
2.05
8.92
13.54
0.17
12.71
15.7
1.05
45.6
IV362
and IV193)
19
156
25x
490
323
67.1
0.47
100.70
0.25
0.87
1.56
9.72
8.71
0.22
13.07
14.1
1.98
47.1
IV26
Monti
25
160
429
39
35
49.3
0.89
99.75
0.40
0.79
1.11
11.69
6.30
0.24
14.60
15.4
2.62
46.6
IV3I6
from Campello
24
118
333
146
85
53.1
0.86
99.67
0.36
1.48
1.88
9.52
7.17
0.20
14.24
15.3
2.82
46.7
IV330
21
119
392
103
56
50.5
0.66
100.11
0.27
1.24
1.42
9.52
6.45
0.23
14.21
14.7
2.27
49.8
IV331
149
932
45
14
55.6
1.6.7
101.0
0.02
0.07
1.79
12.37
7.74
0.18
13.90
19.8
I .0x
44.1
IV21
1I’19;
5
21
7.7
Gd
0
13.2
2.5
6.4
1.3
HY
OL
MT
ILM
AP
3.7
24.2
DI
22.8
AN
NE
20.7
AB
0.3
2.6
2.1
13.7
6.6
15.4
0
32.1
23.8
2.5
1.1
4.7
1.8
10.7
0
17.0
2.7
22.8
31.2
7.2
0
0
4.0
OR
LU
CIPWnorms (wt.%) QZ 0
3.5
0.5
Yb
6.5
DY
Tb
2.2
26.4
49.7
25.4
545
6.6
10
13
37
249
EU
39
Nd
7
80
32
367
Sm
36
80
La
Ba
Ce
56
196
Nb
73
21
32
265
Y
Zr
8
246
8
936
Rb
Sr
20
0.9
4.5
1.9
13.8
0
15.8
0.3
23.1
33.5
5.3
0
24
45
18
280
20
239
38
397
1
0.2
2.1
2.5
34.7
20.4
18.5
0
11.9
6.8
1.2
0
10
17
7
21
7
71
11
146
16
0.3
2.0
2.3
31.8
0.9
9.3
0
31.7
17.3
3.5
0
12
20
8
65
7
62
28
62
3
0.5
3.5
2.0
12.7
0
35.1
4.7
24.0
12.3
4.3
0
14
21
7
70
9
96
25
236
13
0.6
3.7
2.0
1.9
24.8
12.9
0
39.1
9.5
4.5
0
16
29
9
110
13
98
30
263
11
0.6
3.7
2.3
15.7
15.4
14.3
0
28.7
13.1
5.1
0
0.4
2.6
0.96
1.65
5.3
21.4
31.5
12.4
56
14
114
28
163
16
0.4
5.0
2.6
0
23.1
17.2
0
34.8
9.4
4.7
1.0
26
50
18
173
21
204
37
199
62
0.2
5.4
2.6
9.3
13.7
13.3
0
29.0
16.0
8.8
0
31
58
22
214
33
190
30
382
49
0.4
4.3
2.6
0
25.6
12.8
0
30.0
12.0
7.3
3.6
20
33
13
198
14
145
40
186
1
0.4
2.0
2.0
22.0
0
12.7
1.3
45.3
12.6
0.4
0
0.25
1.75
0.6
0.9
2.6
6.9
7.0
<3
49
1
32
19
301
3
7
0.6
0
7
0.4
2.1
2.1
20.0
4.3
11.7
0
46.0
12.0
<3
58
1
29
16
352
In view of the significant IOU grade
il~~t~nl~~h~~n~
trencfs. especially
for thttsr elements
that Ni and Cr correlate suites. It ia not possible pelite and carbonate. the trends element
changes of lwsalt.
atnphibolites probably
to produce although
immobile
modified
high
unmodified.
tr;rcc element
uhich
tmmohilc.
Figure 4 &owh
together
protoliths. and
although
samples
analysed
appearance
have igneous
the IcJL~-~(: part ot
basaltic
111’other trace
rock\ suggests that all these
their major
element
or meta~nmatism.
chemistry
Hone~er.
has
relati\,el\.
rare-earth samples
formed from ~~~tas~dirnent~~r~ precursors of similar
to produce
with the close matching
h> metamorphism
field-strength
for igneoub
this range in Ni and C‘r value:, from mlxturc> of
it would be po\aihle
Some of the Ca-rich
amphibolites
arc rclativclv
etc.. see later) to ccmmon
have igneous
been
I~~~~~~during
to cwkkr
well u ith mg ( IOOhlg,. (Mg 4~ Fe)) ah expected
in Fig. 4. Thib evidence.
ratios (Ti/Zr
in hulk cw~~po.sition ivhloh an it i\ a140 important
element ahundancex appear to he on the plots shown could poa‘;ihl\: have
but. in view of their close association
which must be igneoux, it is inferred
\vith
that all the
parentage.
The mafic granulites from C‘ampello Monti (IV21 and IV193: Table 3) have a distinctly different composition from both the type 1 and type 2 amphibolites and data). It is more closely resemble cumuIates from Val Sesia (Sills. unptiblish~d assumed
that
complex
and will be considered
Before
attempting
protoliths.
reflects
the chemical
affinities
basalt.
in Ca with respect
Figures
to normal
Stephenson and Hensel (1982) for amphibolites Australia. In a total alkalis (Na,O + KzO) versus 1971; Fig. 5a). the amphibolites concentrations
of TiO,.
mafic-uitramafic
of the assumed
to assess the degree to which the present
that of the parent
are enriched
of the Val Sesia layered
no further.
to ascertain
it is first necessary
composition samples
these form an extension
straddle
2 and 3 suggest basal&.
of samples,
that some
a feature
noted
by
from the Wongwibinda belt. SiOz plot (Irving and Baragar.
the tholeiite-alkali
basalt
divide.
Nb and P,O, (Figs. 5c.d) suggest that a tholeiitic
more likely for the majority
basalt
amphibolitr
which implies enrichment
but the parent
i.s
of alkalis during
ln~tarnorphisn~ or low temperature alteration. K,O does not correlate well with other. less mobile incompatible elements, particularly in the type 1 anlphibol~tes. where the high abundance (average 0.7 wt.%) relative to these elements suggests that it has been mobile. Rb and Sr similarly have rather variable concentrations suggesting that they too have been affected, but Ba (Fig. 5b) correlates well with Zr. an at low Ba immobile incompatible element. However, some scatter is apparent contents, suggesting that Ba has been slightly modified, especially in the type 1 amphibolites. The normative plagioclase composition. when compared with the A1,0? content of 13.5-17.0 wt.%. suggests a tholeiitic rather than talc-alkaline parent for all but
197
I
ALKALINE
x
x
-!
/ THOLEIITE
0.
Zr
200
x
46 -
4,8
52 SlOP
5,O
. -
+h+
400
x
200
x
0.
*
. -
x
)l
.
7
.
*x
A 50
a
z&b
Fig. 5. (a) Na,O+
,150
,200
K,O
Ba vs Zr. (c) TiO,
,
Zr
,eJJ
vs SiO,. The alkaline
vs Zr. (d) P,O,
(1975). (e) Y vs Zr. Symbols
,100
,150
and tholeiite
vs Zr, alkaline
,200
,
Zr
fields are from Irving and Baragar
and tholeiite
fields are from Floyd
(1971). (b)
and Winchester
as in Fig. 2.
two Na-rich samples (Irving and Baragar, 1971). These may have suffered secondary enrichment in sodium. A tholeiitic parentage would seem likely for the majority of samples
and the mildly nepheline
normative
nature
of about half the samples
could
result from SiO, depletion. It appears therefore, that the main effects of metamorphism are enrichment in K. Rb and in some cases Na, Ba, Sr and locally Ca, but depletion tion
in SiO,. Low temperature
of Ca through
chlorite.
In many cases calcium
Ca has increased Ca-phase,
the replacement
possibly
hydrous
alteration
of plagioclase
is lost (e.g., Valiance,
in some of these amphibolites; calcite,
of basalt by albite
pyroxenes
by
1974) but Fig. 2 suggests that
presumably
at the time of alteration,
causes redistribuand
the growth
permitted
of a new
redistribution
of
calcium. Trace quantities of calcite are found in one of the Ca-rich samples, IV391A. There are, however, no complementary samples obviously depleted in Ca as described by Stephenson and Hensel (1982). Reported chemical trends resulting from seawater alteration of basalt (e.g., Hart, 1970) and spilitisation (Valiance, 1974) show enrichment in Na, K, Sr, Ba and Rb with loss of Ca and Si which, apart from Ca-depletion, are features shown by the Ivrea Zone samples. Some elements, such as Al, Ti, Zr, Y, and Nb generally remain relatively immobile (Pearce and Cann, 1973; Floyd and Winchester, 1975) except where the degree of alteration is very intensive (Valiance, 1974). As the extent of major element mobility in the Ivrea amphibolites is limited and concentrations of immobile incompatible elements closely match those of unaltered basalts, we can reasonably assume that the concentration of these immobile elements will reflect that of the parent basalt.
As previously alkali
basalt
described,
the amphibol~tes czm be ascribed to either tholeiitic or on the basis of their major element chemistry, hut as the
protoiiths
alkali content
has probably
be placed
on
differences
in trace element
this
(Pearce and Cann,
and
increased
during
it is more
instructive
abundances
1973; Langmuir
metamorphism,
not much reliance
to consider
and ratios between
the well established
basalts of different
et al., 1977; Pearce and Norry.
al., 1979). On the basis of Ti. P. Zr, Nb. Ba. Y and REE contents can be divided into two groups. First the type 1 amphiboiites, Strona
and at Nib&o
elements
(Table
exposed
in Val d’Ossoia
between
Forno
of Ba, are typical
of depleted
Type 2 amphiboiites,
rare-earth oceanic
et
the amphiboiites
and Sambughetto
in Vai of these
from I- 2 wt.%, Zr from 70 - 120 ppm. Nb
< 7 ppm, P,O, from 0.08 to 0.7 wt.%, Y from XI-50 ppm and Ba from 40-100 ‘They all have LREE-depleted
types
1979; Saunders
(Fig. 1). have quite low abundances
2; Fig. 5); TiO, ranging
can
patterns.
ppm.
These values. with the exception
tholeiites.
however, show a much greater range of values with Zr up to
250 ppm, Nb from 7.-33 ppm, TiO? up to 3.4 wt.%, P&I, up to 0.5 wt.% but Y is lower. only ranging up to 40 ppm. Ail samples in this group have LREE-enriched rare-earth
patterns,
One sample,
IV32, has very high Nb (56 ppm)
and
is more
alkaline than any other sample. There is good correlation between pairs of incompatible elements (Fig. 5) and an empirical division between alkaline and tholeiitic basalts
based
on Zr-P,OS
(Floyd
and
samples to have tholeiitic affinities. In Fig. 6, the amphibolites are plotted
(a)
(b)
Winchester.
1975) shows
on a Ti/lOO--Zr-Y
the majority
X 3 diagram
of
iPearce
,
Zr Fig. 6. (a) ‘G/100-22-Y plate basalts.
(b) Zr/Y-Zr
x 3 plot after Pearce
and Cann (1973). OFB-ocean
plot after Pearce and Norry (1979). Symbols
floor basal@,WPB-within as in Fig. 2.
199
and
Cann,
1973) and Zr/Y-Zr
accepted
that even modern
diagram,
the diagrams,
diagram
(Pearce
nonetheless,
provide
Figs. 6a and 6b, the type 1 amphibolites type 2 amphibolites no
tendency
and Norry,
1979). Although
day basalts do not always plot “correctly” some indication cluster
of basalt
in the ocean
type. In both
floor field while the
plot either in the ocean floor or the within plate fields. There is
in any
type
of discriminatory
talc-alkaline field. The type 2 amphibolites, elements, are more intimately
diagram
for samples
to plot
have
Nb
and
TiO,
levels
metasediments
in a
which have enhanced concentrations of incompatible mixed with the metasediments, hence it is necessary to
consider whether these higher levels are primary or the result of chemical with the surrounding metasediments. Many of the type 2 amphibolites, surrounding
it is
on this type of
higher
than
that
of the average
exchange however,
composition
(19 ppm and 1.2 wt.% respectively;
of the
Sills, unpublished
data), suggesting that the concentrations in the amphibolites are primary. In fact Figs. 2 and 3 suggest that it is the type 1 amphibolites which are the most altered. On
the basis
concluded the
type
of these
standard
discrimination
that the type 1 amphibolites 2 amphibolites
represent
diagrams
were depleted more
it can be tentatively
ocean-floor
incompatible
basalts,
whereas
element-enriched
oceanic
basalts and possibly even continental tholeiites. These inferences are further confirmed by examination of various trace element ratios (Table 4). Trace element ratios are more affected
useful
for comparison
by small amounts
of different
of fractionation
basalt
types
than absolute
because
ratios
abundances.
are less
One critical
ratio is Y/Nb which varies markedly between different basalt types (Floyd and Winchester, 1975). As can be seen from Table 4, the type 1 amphibolites, with an average depleted enriched
Y/Nb
ratio of 11.24 + 6.4 (2~) compares
(N-type) (E-type)
with that of a typical
MORB. The type 2 amphibolites are more comparable basalts. The same is true for almost all other element
Atlantic
with more ratios with
TABLE 4 Selected element ratios for the Ivrea Zone amphibolites (data from Tables 2 and 3) and analyses of various basalt types * Av. of
Av. of
Av. of
N-type
T-type
E-type
Alkali
type 1
type 2
378 + 379
MORB
MORB
MORB
basalt
Ba/Zr
0.6
1.0
1.7
Ba/Nb
14.8
8.1
Zr/Nb
25.8
Y/Nb
11.2
-
0.13
0.64
1.71
20.7
4
5.4
7.2
8.3
12.1
30
8.5
4.2
1.8
1.9
11
2.7
0.68
BCR-1
1.7
3.7
10
36
5
9.1
i 1
2.1
Zr/P@,
710
564
574
692
495
533
458
514
Ti/Zr
100
96
60
91
95
60
80
73
* N-type, E-type (enriched) and alkali basalt are taken from Wood et al. (1979a); the T-type (transitional) is from Wood et al. (1979b). data for BCR-1, a continental flood basalt is from Abbey (1980).
the exception
of those involving
been affected
by the metamorphism.
depleted
(N-type)
tholeiites,
MORB.
with more enriched compares
Figure 7 shows selected samples plots (not shown). when normalized can he compared
within
the FAMOUS
From the evidence between
a mid-ocean
to
to island-art:
reasonably
well with
tholeiites. whereas the type 3 dmphibolites c can hr: compared basalta. This wide range in basalt ~(~i~p~~sit~o~ls from Val Stmna
well with the range of composition.\
example
normaiised
for Sr and Nb.
the type 1 amphiholites
ocean-floor
cfepletecl
Similar
show large anomalies
In conclusion,
K. Rb. and Ba, the values for which }~avr pr(,bab]>
available
found in the present
region (Langmuir
day Atlantic,
for
et al.. 1977; Tame\;
et al.. 197%).
from these an~phib~~lites it is not possible
to disting~lish
ridge and back-arc
origin.
The type 1 amphibolites occur within a 2.5 km thick so it seems reasonable to attempt to relate them petrogenetically. Multi-ele-
(a) Tape 1 urnphiholites.
section
ment plots (Fig. 7’1demonstrate their close affinity with MORB in all but thr most lithophile elements. Their REE patterns (Fig. 8) show LREE depletion with Ce,/Yb, anomalies.
ranging from 0.66 to 0.98, and There is a range in REE abundances
they have very slight negative Eu with Ce, ranging from 9 to 17 times 7
Fig. 7. (a) Selected Attantic
N-type
type 1 amphibolites
and (b) selected
MORB (Wood et al., 1979a).
type 2 ampbibolites
normalised
to a typical
201 Type
5
1’ La Ce
1
Nd
f
Amphibolltes
I I
““1
Yb Lu
Sm Eu Gd Tb Dy Type
2
Amphibolltes
3
5u-.--kkLa Ce Pr Nd
Sm Eu Gd Tb Dy
Fig. 8. Chondrite normalised REE plots for representative type 1 and type 2 amphiboIites.
and Yb from
chondrite
the shape
of
the
decreasing
mg (sample
9 to 21 times chondrite
REE
patterns.
This
IV386 having
value of 49.1) and with increasing
in REE
an mg number
content
of a phase which has negligible
most
As shown
k,
in Fig. 4, the marked
change in
correlates
with
of 64.4 while IV201 has an mg
levels of other incompatible
fractionation likely.
but there is no significant
increase
elements.
for these elements, positive
correlation
This implies
olivine being the of Ni with mg
confirms the possibility of olivine fractionation. The negative Eu anomalies, coupled with the low Sr values (Fig. 7) suggests plagioclase has been removed. The relatively constant
ratios of Ti/Zr
(average
loo), Zr/Y
(2.5) and Zr/Nb
(26) suggest all these
elements remained incompatible during fractionation, eliminating clinopyroxene and hornblende as major fractionating phases. These data suggest that the most mg-rich samples,
such as IV386,
either
represent
a primary
liquid,
the remaining
liquids
developing by removal of olivine and possibly plagioclase, or that the most mg-rich samples contained cumulus olivine. As the rocks are completely metamorphosed and no igneous The range plagioclase (b) Type
textures remain, it is impossible to distinguish between these possibilities. of REE and high field strength elements is compatible with olivine and fractionation, which implies fractionation at relatively low pressures. 2 ~mp~ib~~i~es. As previously discussed the type 2 amphibolites appear to
have been less affected by low-temperature alteration but the range in composition is much greater. In view of this wide compositional range and the larger area over
which
the amphibolites
were sampled.
it is not realistic
(XRF data). C’e,,‘Yh,
The tectonic considerable
Of. ‘1Ht. .1MPHIROl.I I1.S
evolution
the origins
of the Ivrea Zone is p<)orly understood.
of the amphiholitts
bearing
to relate them
(Fig. Xt ranges from 3.6 tcl .?.I.
‘I b.C’TONIC‘ SIGNIFIC‘AN(‘1,
regarding
t
to the type 1 ~~t~ipllib~)lit~s have (‘~1, Y.. -- 1
p~trogel~~ti~al~y. All samples. in contrast
on this problem.
with the enclosing
There is no evidence
hence information metasediments
as to the nature
has a of anv
basement upon which a sedimentary sequence may have been deposited. Geophysical data suggest that rocks of mantle density approach the surface under the Ivrea Zone (Berckhemer, 1969). but there is no geophysical evidence for continental basement. The metasedimentt amphibolite zone may of course be allochthonous. It is also possible that the ~nterbandi~lg of ~lrnphib~~~ite and tlletased~ment is ;I primary feature of the processes (of crustal growth that may take place at a continental margin
during
development
of an accretionary
The metasediments are dominantly ranging from 52 to 70 wt.%, (Mehnart.
sedimentary
wedge.
composed of quartz-rich pelite with SO, 1975: Sills, unpublished data) and possibly
representing immature sediments such as greywacke (Mehnart. 1975 ). The atnphibolite-facies metasediments are extremely highly deformed so that all bedding has been completely
transposed
are numerous
and
and no original
have also been
bedding
transposed
features into
remain.
narrow
Small quartz
lenses
parallel
veins to the
foliation. The amphibolites. whatever their origin. have been similarly boudinaged into a series of lenses parallel with the regional foliation. The large Mafia-ultramafi~ complex (Fig. 1) intrudes the metasediments but is itself relatively undeformed. with igneous textures and cumulate structures locally preserved (Rivalenti et al.. 1975). The sediments are also intruded by Hercynian granitoids (Fig, 1). This means that the very intense deformation suffered by the metasediments and intercalated amph~boiites predates the emplacement of the mafic complex and is thus of Hercynian or earlier age. There is still uncertainty as to the age of the sediments and their metamorphism. Hunziker and Zingg (1980) obtained a Rh/Sr isochron of 475 of:20 Ma from 30 to 50 kg samples which they have interpreted as the age of granulite facies metamorphism and suggest sedimentation at 500- 700 Ma. However. zircon ages suggest metamorphism at a younger age. perhaps (K$@, 1974; Pm and Vielzeuf, 1983). although interpretation
as young as 300 Ma is subject to ambigu-
ity. The fate of sediments on the oceanic plate during subduction is of considerable interest. It is known that in some cases, such as the Mariana Island Arc. the sediments may be almost entirely subducted forming little or no accretionary wedge (Rarig and Kay, 1981). Even where there is an additional influx of continental-derived sediments, such as the Middle America Trench (Watkins et al., 1982) or the Japan trench (Von Huene et al., 1980), there may be more sediment subducted than accreted, provided that the continental sedimentation rate is relatively low. However,
203
where the sediment subducting
supply
from the continent
plate so that it approaches
are accreted
the trench
back on to the continental
1980). In southern
margin
Chile a major accretionary
late Palaeozoic-early and is exposed
Mesozoic (Forsythe,
on the trenchward
side of the present
made up of continent but is extremely
planes
dipping
at shallow
angles
towards
Moore
wedge developed
with minor
the
at a low angle and the sediments (cf. Alaska,
derived
deformed
and Allwardt,
in this way during
1982; Bartholomew
wedge is predominantly carbonates,
is higher, the effect is to depress
and Tarney.
Patagonian
Batholith.
greywacke-shale along
the continent.
the
in press). The
components
closely-spaced
All bedding
foliation
is completely
transformed and quartz veins, also transposed within the foliation. are abundant. The rocks are very similar to those in Val Strona. The Franciscan complex of California
(e.g., Ernst,
contains
1975). although
components
of oceanic
sediments. During
the formation
thickness
to consist
of an accretionary
predominantly
pre-existing
continental
ocean
or juxtaposed
crust
tectonic ocean
environment crust
basement.
wedge, it is possible
The accreted peridotitic
without
sediments mantle
quartz
stripped
grade, similarly
intermixed
with
immature
for the whole crustal the necessity
off and incorporated
plate.
the large
talc-alkaline
mafic
affinities
body
into
the accreting
deformation (Ernst, 1975; Karig, 1982). Low-grade and suffer dehydration, which leads to the develop-
veins in the overlying
in Val Sesia,
(Rivalenti
The fact that the amphibohtes MORB
and more enriched
on The
from the subducting
rocks.
Contemporaneous
kaline igneous activity may lie well to the continent side of the accreting Although there are no talc-alkaline volcanics in Val Strona. it is interesting that
of a
may be superimposed
of the overriding
is one where slivers of mafic volcanics
may be continually
of abundant
tectonically
of metasediment
against
wedge and subject to continual metasediments are unde~lated ment
much lower in metamorphic
affinity
which
intrudes
et al., 1975; Bigioggero in VaI Strona
tholeiite
or alkalic
wedge. to note has
et al., 19781979).
appear basaltic
the metasediments,
calc-al-
to represent compositions
both N-type is consistent
with an accretionary wedge model. The floor of most oceans is irregular due to the presence of seamounts, fracture zones and transform faults, many of which are the sites of alkali or enriched type MORB. It is these irreguIarities which are more Iikely to be stripped off and incorporated in an accretionary wedge during subduction. The rock type kinzigite may represent metasediment sive deformation in an accretionary wedge, followed
which has undergone pervaby high-grade post-deforma-
tional metamorphism as temperatures approached the equilibrium geotherm. Stronalite is simply the granulite facies equivalent. In Val Strona, stronahtes occur close to the mafic-ultramafic complex and the granulite facies metamo~hism is probably related to its emplacement (Schmid and Wood, 1976). This is further confirmed by the relatively low pressures of metamorphism of about 6-6.5 kbar (Newton and Haselton, 1981).
‘L‘ONC‘I.USIONS
(1)
Amphibolitrs
Zone
(2) On two
occurring
have igneous
rather
the basis
groups;
type
of their
ocean basalt
(3) The
(4)
occurrence
with
metasedinrentar>
formation
of
the
fvrra
they can he divided
into
prc~tolithx.
and cconpoaition,
with
strong
affinities
to depleted
a more varied composition
equivalent
N-type
MORB
to more enriched
compositions.
range in composition
by olivine
the
sedimentary
1 amphibnlites
and type 2 amphibolites (E-type)
w(thin
than
of the type 1 ~in~phih~~lit~s
suggests
It
fractj~~nat~~n. that this section of the crust was huiit
they are related
and FIagioclase is possible
wedge during
up ah an accretionary
the early to mid-Palaeozoic.
ACKNOWLEDGEMENTS
JDS gratefully acknowledges the receipt of an NERC post-doctoral research fellowship. We would like to thank Phil Potts and Olwen Thorpe at the Open University
for the INAA
We are very grateful
data and Nick Walsh at Kings College
for the assistance
of Giorgio
Rivalenti
for the ICP data.
in the field.
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Abbey,
S.. 1980. Studies
minerals.
Geostand.
Bartholomew,
in “standard Newslett.,
D.S. and Tarney.
samples”
J., in press. Geochemtcal
Andes (45 ’ -46 OS). In: R.S. Harmon Isotopic
Constraints,
Berckhemer.
IL.
Bigioggero, farm,
Central
(Editors),
of magmattsm
Andean
evidence
for
the
composition
of
the
A.. Colombo.
A. and Gregnanin.
A.. 197X-1979.
Alps, Italy. Mem. Inst. Geol. Min. Univ. Padua.
S.. Coradini,
Ivrea-Verbano
of stlicate
rocks
and
rn the southern
Magmatism:
Chemical
and
Ltd., Nantwich.
J.R., 1969. Spilites from the Cartsberg
Capedri.
analysis
lower
crust
and
the
Moho.
8: 97-105.
EL, Boriani.
complex,
Direct
characteristics
and B. Barreiro
Shiva Publications
1969.
Tectonophysics.
for use in the general
4: 163.-190.
A., Fanucci.
ridge. Indian
0.. Garuti.
basic formation
(ltalian
Ocean.
of the Ivrra basic
J. Petrol..
IO: l-19.
G. and Rossi. A.. 1977. The origin of the
G., Rivalenti.
Western
The diorites
33: 71-85.
Alps). Statistical
approach
to the peridotite
problem.
Rend. Cont. Sot. It. Miner. Petrol.. 33: 5833592. Dostal.
J. and Capedri.
Alps. Lithos. Ernst,
W.G..
1975. Introduction.
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Field
and
PETROGENESIS INTERLAYERED
B.V.. Amsterdam
(Received
of Geolqy,
November
in The Netherlands
ALPS, NORTHWEST
D. SILLS and JOHN
Department
- Printed
AND TECTONIC SIGNIFICANCE OF AMPHIBOLITES WITH METASEDIMENTARY GNEISSES IN THE IVREA
ZONE, SOUTHERN
JANE
187
107 (1984) 187-206
Elsevier Science Publishers
ITALY
TARNEY
Uniuersity
of Leicester, Lercester. LEI 7RH (United Kingdom)
7, 1983; revised version accepted
February
27. 1984)
ABSTRACT
Sills, J.D. and Tarney, metasedimentary
J., 1984. Petrogenesis gneisses
and tectonic
in the Ivrea Zone,
significance
Southern
of amphibolites
Alps, Northwest
Italy.
interlayered
with
Tectono&sics,
107:
187-206. Amphibolites formation
occurring
of the lvrea
characteristics
as elongate
Zone
(N-type)
have higher basalts.
and
mid-ocean
distributions
ridge basalts.
metamorphism
showing
appears
elements
basaltic
The tectonic formed
geochemical
deformed
at a (Palaeozoic)
study
of their
to transitional the rare-earth
elements
typical
layers and boudins
or enriched
in loss of SiO, and variable
including
element
occur as layers up to
in incompatible
occur as thinner
(kinzigite) trace
(E-type)
enrichment
elements,
of and
ocean in Na.
have preserved
characteristics.
model which best explains
highly
the metasedimentary
Type 1 amphibolites
depletion
similar
to have resulted
K. Rb, Ba, Sr and locally Ca, the less mobile elements,
extremely
within
A detailed
Type 2 amphibolites
levels of the more incompatible
Although
the primary
boudins
parentage.
shows that they can be divided into two groups.
100 m thick and have trace-element normal
lenses
are all of igneous
semi-pelitic
subduction
the assemblage
metasediments
of slivers of MORB-like is that
of an accretionary
amphibolites
within
sedimentary
wedge
zone.
INTRODUCTION
The occurrence of amphibolite layers within metasedimentary sequences poses problems as to their origin, particularly where the rocks are so deformed that any original intrusive or sedimentary relationships have been totally destroyed. Such is the case with amphibolites occurring within the metasedimentary gneisses of the Ivrea Zone of the Southern Alps in northwest Italy (Mehnart, 1975). This region is of special interest because it is commonly regarded as a section of the deeper crust (Mehnart, 1975; Fountain and Salisbury, 1981) and therefore can provide information on the processes of crustal development. Amphibolites may be derived from basaltic rocks, from calcareous or dolomitic
0040-1951/84/$03.00
0 1984 Elsevier Science Publishers
B.V.
lxx
shales (Leake, and
carbonate
impossible
1964). or develop bands
to produce
to determine
thin
their origin
must be made to geochemical is unlikely
through
layers
exchange
(Orville,
Although
and significant
occurred, Leake (1964) has shown that primary cal trends may be preserved, thus permitting para-amphibolites.
between
adjacent
1969). In many
from field or textural
techniques.
to have been isochemical
chemical
observations.
metamorphism compositional
pelite
cases it is so recourse
of amphibolites change
may have
magmatic or sedimentary geochemidiscrimination between ortho- and
If the amphibolites have a basaltic protolith. it is vital to assess the extent of low-temperature alteration which is known to have a major effect on the composition (e.g.. Hart, 1970; Cann, 1969). However, several studies have shown that some high field-strength elements (Ti. Zr, Nb, Y) remain relatively immobile during metamorphism and may be used to indicate the original basalt type and possible tectonic setting (Pearce and Cann. 1973: Floyd and Winchester, 1975: Stephenson and Hensel, 1982). This study investigates the geochemistry of the amphibolites and considers their origin and likely tectonic setting which may bear upon the evolution of the Ivrea Zone as a whole. GEOLOGICAL
SE’MING
The Ivrea Zone (Fig.
1) consists
of a steeply
dipping
series of metabasites
and
metasedimentary gneisses of Palaeozoic age inferred by many authors to be a deformed section through the lower continental crust (e.g. Mehnart. 1975). The metasediments (termed “kinzigite” at amphibolite facies and “stronalite” at granu-
Fig. 1. A. Sketch map of the Ivrea Zone. 1980.) Crosses amphibolites: layered
B. Sketch map of Val Strona and Val d’ossola. (After Zingg. IV28. 29, 30, 200, 201. 386. 390, 391 and 391A are type 1 IV21 and 193 are related to the IV25. 26, 32, 199. 378. 379 and 382 are type 2 amphibolites: mark sample localities.
complex.
Samples
189
lite facies) are a heterogeneous Metamorphosed
mafic
mixture
rocks make
of pelite, semi-pelite
up a significant
and carbonate
proportion
rocks.
of the Ivrea Zone
(Fig. 1) and are of two kinds: (a) a large
layered
mafic-ultramafic
Finer0 (Rivalenti et al., 1975) and (b) layers of amphibolite within
complex,
the metasediments.
occurring
in Val Sesia and
It is the amphibolites
at
which
form the subject of this paper, representatives of which are best exposed in Val Strona (Fig. 1). The metamorphic grade increases from amphibolite facies in the southeast to granulite facies in the northwest (Mehnart, 1975; Schmid and Wood, 1976; Hunziker and Zingg, 1980) and the prograde petrological changes in the metabasites have been described in detail by Reinsch (1973a,b); Mehnart (1975) and Zingg (1980). The mafic rocks investigated in this study are of three types: (a) Units of amphibolite, up to 100 m thick but commonly lo-20 m thick, interbanded with garnet-biotite gneiss occurring between Forno and Sambughetto in Val Strona and at Nibbio in Val d’Ossola (Fig. 1). These will be referred to as type 1 amphibolites. (b) Thin layers or lenses, 25-100 cm thick, complexly interfolded with the sediments. These are common in the amphibolite facies zones but also occur in the granulite
facies zones. These wiI1 be referred
to as type 2 amphibolites.
(c) Garnetiferous metabasites occurring at the base of the section at Campello Monti. These are continuous with the pyroxenites and metagabbros of Val Sesia (Capedri et al., 1977) but have a quite different composition from the other metabasites in Val Strona (Dostal and Capedri, 1979). The assumption that all the metabasites in the Ivrea Zone have a common origin (Mehnart, 1975: Zingg, 1980) is not necessarily correct. This paper concentrates on the bulk rock major and trace element chemistry of these
different
metabasic
rocks,
petrography, mineral chemistry elsewhere (e.g., Zingg, 1980).
and
is prefaced
and metamorphism,
by only
limited
discussion
of
as these have been described
In marked contrast to the gabbros from the layered complex, the amphibolites are medium- to fine-grained, well foliated rocks containing alternating felsic and mafic bands.
The amphibolites
in the lower part of Val Strona
comprise
50-70s
green
prismatic hornblende, 20-40% polygonal plagioclase, up to 10% biotite with accessory quartz, apatite and sphene (which usually rims ilmenite). As described by Reinsch (1973a), the colour of the hornblende changes systematically from green to brown as the metamorphic grade increases. The type 1 amphibolites contain up to 30 vol% of a pale green to colourless anhedral clinopyroxene. This has very low TiO,, AI,O,, (< 1.5 wt.%) and Cr,O, contents and has the approximate composition Ca 48-soMg83-.t6Fe15-,6. The clinopyroxene-bearing rocks are banded with pale
190
cl~nopyroxene-plag~~lase
and
darker
hornblende-rich
layers.
Accessory
minerals
include seapolite, sphene. opaque oxides, apatite, green biotite and carbonate. At Nibbio there are 10 cm long pegmatoid lenses containing scapolite
(Me,, ). plagioclase.
(andradite
54%. grossular
Near
Forno
sphene
and
unusual
apricot-coloured
garnet
1 wt.% TiO, ).
33%. with about
in Val Strona.
an
traces of epidate,
a 1 m wide ultramafic
layer consisting
of brown
hornblende, orthopyroxene and olivine occurs. Mafic granulite layers have a polygonal eyuigranular texture consisting of plagioclase, orthopyroxene. clinopyroxene and brown
hornblende
granulites
with accessory
at Campello
Monti
red biotite.
opaque
are garnetiferous
oxides and apatite.
and
pyroxene and plagiociase up to 0.5--l cm across 1 cm across but up to 2.5 cm across.
much
and
coarser
euhedral
The mafic
grained,
garnets
with
generally
0.5
CHEMISTRY
AND PETROGENESIS
Analyses were performed using a Philips PW1400 X-ray fluorescence spectrometer at the University of Leicester following analytical techniques outlined by Tarney et al. (1979a), Marsh et al. (1980) and Weaver et al. (1983). Briefly, major elements were determined on glass discs fused with a lithium tetraborate flux. and trace elements on pressed powder pellets. Major elements, Ni, Zr. Y, Zn, Sr, Rb and Ga were analysed using a rhodiun~ target and Cr. Ti, V, Ba, La, Ce and Nd using a
TABLE Analyses
1
(ppm) of Whin Sill. an Open
amphibolites.
1X1.
University “ill house” standard -I_lV147 -.-~ INAA
Whin Sill
Elem.
run
wth
the Val Strona
IV26. IV28 and IV201 *
fCP(7)
ICP(2)
~.--_ll_____-La
meaa.
recom.
25.0
25.0
19.8
19.8
21.3
<‘c
58.0
57.6
47.5
42.3
44.7
Nd
il.6
72.3
31.1
‘7.3
24.7
Sm
7.3
7.2
4.5
4.7
4.4
ELI
2.26
2.19
4.1
3.4
3.7
Cd
6.X
7.0
4.6
i.8
4.4
‘l-h
1.12
1.0x
0.5.:
Tm Yh
0.45 1.52
0.42 2.4x
0.64
0.6X
(1.74
1.11
0.38
0.37
0.08
0.11
0.14
* The recommended
value is an average of repeated
and JCP (twice) IO show the comparability analyses
is about
5%. for La. 10% for Cr.
analyses.
> 25% for Gd and Tm. For the ICP method
the analytical
the chemical
introduces
separation
technique
probably
IV147 is a sample analysed
hy both INAA
of the two techniques. The analytical precision for the lNAA < 5% for Sm. ELI. Tb and Yb, about 15% for Lu and Nd and precision
further
is I-6% (Walsh et al., 1981) although
errors.
191
TABLE
2
Type 1 amphibolites. Components
Val Strona and Val d’Ossola
Rock samples: IV28
IV28
IV30
IV200
IV201
46.6
47.3
48.4
IV380
IV386
IV390
46.0
46.3
IV391
IV391A
(wt.%)
Chemical ana!vses
47.8 1.92
0.95
1.45
1.97
48.8 1.89
TiO’ SiO
47.1 1.32
Al z;,
17.3
17.0
16.1
16.2
14.7
14.6
Fe203
0.98 16.8
1.72
47.3 1.79
45.9 1.78
13.4
13.9
14.0
11.12
14.35
10.34
10.54
13.76
13.35
9.64
13.33
13.50
13.88
MnO
0.18
0.22
0.18
0.16
0.22
0.21
0.16
0.27
0.19
0.23
MgO
7.06
6.19
11.02
7.28
5.90
6.34
7.76
5.82
6.68
6.17
CaO
13.29
9.22
13.64
14.78
11.60
11.47
16.50
16.27
12.18
15.43
NazO
2.22
3.47
0.99
2.18
2.62
2.31
1.64
2.63
3.35
2.46
K2O
0.86
0.63
0.86
0.72
0.74
0.67
0.63
0.35
0.92
0.44
p205
0.08
0.17
0.06
0.11
0.17
0.17
0.10
0.15
0.15
0.19
100.53
100.97
100.74
100.72
100.08
99.81
100.21
100.24
99.96
100.48
0.69
0.93
1.25
0.52
0.36
0.41
0.74
1.43
0.75
0.79
Total LO1 %
58.8
49.3
70.6
60.8
49.1
51.7
64.4
49.6
52.7
50.0
51
Trace elements @pm) Ni
96
63
299
153
68
70
226
45
50
Cr
298
276
494
345
199
223
598
90
87
96
V
263
352
164
242
663
374
185
321
348
318
Zn
106
129
86
84
116
117
77
93
108
108
Ga
17
23
15
18
20
18
14
19
18
20
Rb
21
7
17
11
6
7
11
4
11
3
Sr
108
157
189
177
125
122
137
171
96
127
Y
30
55
23
30
44
43
25
42
41
48
Zr
72
98
65
102
118
115
59
107
106
118
Nb
3
6
3
4
6
3
2
3
3
3
Ba
57
39
105
48
38
60
46
34
40
38
-cl
3
5.3
4
3.8
3
4.0
4
Ce
8.5
20
7
9
14.9
9
7.7
11
10.8
11
Nd
9.3
17
5
10
14.4
9
6.0
9
8.3
9
Sm
3.4
La
EU Gd Tb
5
<3
1.3 nd
4.8
2.4
1.6
0.8
1.2
3.0
4.8
3.3
4.8
nd
3.3
0.8
1.25
Yb
3.26
4.7
2.0
3.0
LU
0.5
0.71
0.33
0.49
DY
CIP W norms (wt.%) QZ
0
0
0
0
0
0
0
0
0
0
OR
5.1
3.7
5.0
4.2
4.4
4.0
3.7
2.1
5.4
2.6
AB
13.7
27.4
8.3
11.0
22.1
19.6
3.7
7.7
15.9
8.6
AN
34.5
28.7
36.7
32.1
26.1
27.5
36.5
23.7
20.2
25.7
Kc,ck wlple\: 11’2X N F.
I)1
IV200
IV28
2.7
0.0
3.0
71.1
12.9
32.7
IV380
IV.201 0
I\‘?‘11
IV?L)IA
5.5
(> 7
36.3
32.5
40.x
iI
0
0
7.5
‘.I
I\.‘?Y(l
I V?Xh
(3.6
H\i
0
0
0
4.:
13.4
OI-
13.4
1x.7
IN?
0.x
4.5
9.7
1141
S.?
M-l-
2.0
2.6
I .9
7.1
7.4
I7
14
2.5
ILM’I
2.5
3.6
7.7
17
.J.h
I.‘)
i.4
3.4
Ap
0.2
0.4
0.3
0.3
0.4
0.2
(I.4
(I.4
(1
tungsten target. Additional rare earth data were obtained for four samples (IV21, IV26, IV28 and IV201) by instrumental neutron activation analysis at the Open University
using
the method
IV386 and IV391)
of Potts et al. (1981) and
by inductively
London
using
(IV147).
analysed
techniques
Analyses
of type 1 amphibolites
using
coupled
described both
lites as well as metagabbros
by
techniques,
plasma Walsh
for three samples
spectrometry et al. (1981).
(IV378.
at Kings The
gives very comparable
College,
same
results
sample
(Table
1).
are given in Table 2 and those of type 2 amphibo-
from C’ampello Monti
are given in Table
3.
Nature of purent rocks
Chemical analyses istry the amphibolites
(Tables 2 and 3) show that, from their major element resemble aikalineeolivine basaits or olivine tholeiites;
chemabout
c Sphene
OPX
CPX
Fig. 2. ACF diagram for Val Strona amphibolites.
Field of basalts taken from Orvllie (1969).
this and subsequent figures are as follows:
+ = type 1 amphibolites.
of the most incompatible
amphibolites
element-enriched
x
=
from Sambughetto
type 2 amphibolites. (IV32.
IV378.
Symbols on l = three
IV379).
193
Fig. 3. Niggli 100 mg-c-(al-a/k)
half the samples rarely
quartz
plot within
and c-100
being
nepheline
normative.
mg diagrams
normative,
after Leake (1964). Symbols
the remainder
but with a spread
towards
However, a broad similarity with basaltic compositions origin as any rock consisting dominantly of hornblende its origin, will have a similar Chemical
trends
hypersthene
An ACF plot (Fig. 2) shows that the majority
the field of basalt,
para-amphibolites
being
as in Fig. 2
provide (Leake,
major element
compositions.
does not prove an igneous with plagioclase, whatever
chemistry.
the best means 1964). Figure
Ca-rich
or
of samples
of discriminating
3 shows
that
between
the Niggli
c-mg
ortho-
and
and
100
mg-c-( al-alk ) trends for all the amphibolites broadly follow an igneous rather than a sedimentary trend but there is a spread towards Ca-rich compositions for the clinopyroxene-bearing
type 1 amphibolites,
which does not eliminate
origin for some of this group. The type 2 amphibolites
a sedimentary
follow the igneous
trend more
closely. Ni
Cr
A
10
,40
50
80
70
Fig. 4. Cr vs mg and Ni vs rng (100 Mg/(Mg+ pelites is from Leake (1964).
1 t
,40
50
Fe2+ )) for amphibolites.
80
70
Symbols
as in Fig. 2. Field of
81
123
310
12s
24
Ni
Cr
V
Zn
Ga
Trace elemenrs lppm)
50.1
0.71
LO1
mg
100.37
Total
19
93
277
271
100
59.0
0.45
100.45
0.11
0.57
p205
2.82
10.44
0.42
11.26
CaO
7.60
3.26
4.47
MgO
0.16
11.90
0.68
0.19
MnO
K,O
14.17
Fe@.?
16.9
1.40
48.7
IV376
_
Na,O
14.5
3.37
45.9
AI 1%
TiO,
SO,
Chemica! anu~vsbs (wt.4 )
IV32
Rock samples:
of type 2 amphibolites
3
Components
Analyses
TABLE
21
91
21
248
X8
210
87
56.3
0.88
99.64
0.40
0.90
252
1x4
81
53.6
0.88
100.01
0.45
1.22
4.01
9.00
9.35
4.27
6.20
0.15
10.82
16.0
2.36
49.8
lV379
5.23
0.13
10.18
16.7
2.48
50.0
IV378
9
105
144
1976
1076
79.0
2.68
100.04
0.10
0.20
0.80
7.82
23.98
0.17
14.36
5.9
1.11
45.8
IV382
together with two samples (IV21
15
79
253
684
354
70.6
0.45
17
96
206
509
237
64.7
0.34
101.87
0.22
0.13 100.46
0.74
2.51
13.94
9.06
0.14
11.10
13.8
1.86
47.5
IV199
.71.
85
263
427
76
63.6
0.97
100.59
0.26
0.77
1.13
11.48
7.74
0.17
11.22
17.1
1.95
47.8
IV25
of metagabbroa
0.59
2.05
8.92
13.54
0.17
12.71
15.7
1.05
45.6
IV362
and IV193)
19
156
25x
490
323
67.1
0.47
100.70
0.25
0.87
1.56
9.72
8.71
0.22
13.07
14.1
1.98
47.1
IV26
Monti
25
160
429
39
35
49.3
0.89
99.75
0.40
0.79
1.11
11.69
6.30
0.24
14.60
15.4
2.62
46.6
IV3I6
from Campello
24
118
333
146
85
53.1
0.86
99.67
0.36
1.48
1.88
9.52
7.17
0.20
14.24
15.3
2.82
46.7
IV330
21
119
392
103
56
50.5
0.66
100.11
0.27
1.24
1.42
9.52
6.45
0.23
14.21
14.7
2.27
49.8
IV331
149
932
45
14
55.6
1.6.7
101.0
0.02
0.07
1.79
12.37
7.74
0.18
13.90
19.8
I .0x
44.1
IV21
1I’19;
5
21
7.7
Gd
0
13.2
2.5
6.4
1.3
HY
OL
MT
ILM
AP
3.7
24.2
DI
22.8
AN
NE
20.7
AB
0.3
2.6
2.1
13.7
6.6
15.4
0
32.1
23.8
2.5
1.1
4.7
1.8
10.7
0
17.0
2.7
22.8
31.2
7.2
0
0
4.0
OR
LU
CIPWnorms (wt.%) QZ 0
3.5
0.5
Yb
6.5
DY
Tb
2.2
26.4
49.7
25.4
545
6.6
10
13
37
249
EU
39
Nd
7
80
32
367
Sm
36
80
La
Ba
Ce
56
196
Nb
73
21
32
265
Y
Zr
8
246
8
936
Rb
Sr
20
0.9
4.5
1.9
13.8
0
15.8
0.3
23.1
33.5
5.3
0
24
45
18
280
20
239
38
397
1
0.2
2.1
2.5
34.7
20.4
18.5
0
11.9
6.8
1.2
0
10
17
7
21
7
71
11
146
16
0.3
2.0
2.3
31.8
0.9
9.3
0
31.7
17.3
3.5
0
12
20
8
65
7
62
28
62
3
0.5
3.5
2.0
12.7
0
35.1
4.7
24.0
12.3
4.3
0
14
21
7
70
9
96
25
236
13
0.6
3.7
2.0
1.9
24.8
12.9
0
39.1
9.5
4.5
0
16
29
9
110
13
98
30
263
11
0.6
3.7
2.3
15.7
15.4
14.3
0
28.7
13.1
5.1
0
0.4
2.6
0.96
1.65
5.3
21.4
31.5
12.4
56
14
114
28
163
16
0.4
5.0
2.6
0
23.1
17.2
0
34.8
9.4
4.7
1.0
26
50
18
173
21
204
37
199
62
0.2
5.4
2.6
9.3
13.7
13.3
0
29.0
16.0
8.8
0
31
58
22
214
33
190
30
382
49
0.4
4.3
2.6
0
25.6
12.8
0
30.0
12.0
7.3
3.6
20
33
13
198
14
145
40
186
1
0.4
2.0
2.0
22.0
0
12.7
1.3
45.3
12.6
0.4
0
0.25
1.75
0.6
0.9
2.6
6.9
7.0
<3
49
1
32
19
301
3
7
0.6
0
7
0.4
2.1
2.1
20.0
4.3
11.7
0
46.0
12.0
<3
58
1
29
16
352
In view of the significant IOU grade
il~~t~nl~~h~~n~
trencfs. especially
for thttsr elements
that Ni and Cr correlate suites. It ia not possible pelite and carbonate. the trends element
changes of lwsalt.
atnphibolites probably
to produce although
immobile
modified
high
unmodified.
tr;rcc element
uhich
tmmohilc.
Figure 4 &owh
together
protoliths. and
although
samples
analysed
appearance
have igneous
the IcJL~-~(: part ot
basaltic
111’other trace
rock\ suggests that all these
their major
element
or meta~nmatism.
chemistry
Hone~er.
has
relati\,el\.
rare-earth samples
formed from ~~~tas~dirnent~~r~ precursors of similar
to produce
with the close matching
h> metamorphism
field-strength
for igneoub
this range in Ni and C‘r value:, from mlxturc> of
it would be po\aihle
Some of the Ca-rich
amphibolites
arc rclativclv
etc.. see later) to ccmmon
have igneous
been
I~~~~~~during
to cwkkr
well u ith mg ( IOOhlg,. (Mg 4~ Fe)) ah expected
in Fig. 4. Thib evidence.
ratios (Ti/Zr
in hulk cw~~po.sition ivhloh an it i\ a140 important
element ahundancex appear to he on the plots shown could poa‘;ihl\: have
but. in view of their close association
which must be igneoux, it is inferred
\vith
that all the
parentage.
The mafic granulites from C‘ampello Monti (IV21 and IV193: Table 3) have a distinctly different composition from both the type 1 and type 2 amphibolites and data). It is more closely resemble cumuIates from Val Sesia (Sills. unptiblish~d assumed
that
complex
and will be considered
Before
attempting
protoliths.
reflects
the chemical
affinities
basalt.
in Ca with respect
Figures
to normal
Stephenson and Hensel (1982) for amphibolites Australia. In a total alkalis (Na,O + KzO) versus 1971; Fig. 5a). the amphibolites concentrations
of TiO,.
mafic-uitramafic
of the assumed
to assess the degree to which the present
that of the parent
are enriched
of the Val Sesia layered
no further.
to ascertain
it is first necessary
composition samples
these form an extension
straddle
2 and 3 suggest basal&.
of samples,
that some
a feature
noted
by
from the Wongwibinda belt. SiOz plot (Irving and Baragar.
the tholeiite-alkali
basalt
divide.
Nb and P,O, (Figs. 5c.d) suggest that a tholeiitic
more likely for the majority
basalt
amphibolitr
which implies enrichment
but the parent
i.s
of alkalis during
ln~tarnorphisn~ or low temperature alteration. K,O does not correlate well with other. less mobile incompatible elements, particularly in the type 1 anlphibol~tes. where the high abundance (average 0.7 wt.%) relative to these elements suggests that it has been mobile. Rb and Sr similarly have rather variable concentrations suggesting that they too have been affected, but Ba (Fig. 5b) correlates well with Zr. an at low Ba immobile incompatible element. However, some scatter is apparent contents, suggesting that Ba has been slightly modified, especially in the type 1 amphibolites. The normative plagioclase composition. when compared with the A1,0? content of 13.5-17.0 wt.%. suggests a tholeiitic rather than talc-alkaline parent for all but
197
I
ALKALINE
x
x
-!
/ THOLEIITE
0.
Zr
200
x
46 -
4,8
52 SlOP
5,O
. -
+h+
400
x
200
x
0.
*
. -
x
)l
.
7
.
*x
A 50
a
z&b
Fig. 5. (a) Na,O+
,150
,200
K,O
Ba vs Zr. (c) TiO,
,
Zr
,eJJ
vs SiO,. The alkaline
vs Zr. (d) P,O,
(1975). (e) Y vs Zr. Symbols
,100
,150
and tholeiite
vs Zr, alkaline
,200
,
Zr
fields are from Irving and Baragar
and tholeiite
fields are from Floyd
(1971). (b)
and Winchester
as in Fig. 2.
two Na-rich samples (Irving and Baragar, 1971). These may have suffered secondary enrichment in sodium. A tholeiitic parentage would seem likely for the majority of samples
and the mildly nepheline
normative
nature
of about half the samples
could
result from SiO, depletion. It appears therefore, that the main effects of metamorphism are enrichment in K. Rb and in some cases Na, Ba, Sr and locally Ca, but depletion tion
in SiO,. Low temperature
of Ca through
chlorite.
In many cases calcium
Ca has increased Ca-phase,
the replacement
possibly
hydrous
alteration
of plagioclase
is lost (e.g., Valiance,
in some of these amphibolites; calcite,
of basalt by albite
pyroxenes
by
1974) but Fig. 2 suggests that
presumably
at the time of alteration,
causes redistribuand
the growth
permitted
of a new
redistribution
of
calcium. Trace quantities of calcite are found in one of the Ca-rich samples, IV391A. There are, however, no complementary samples obviously depleted in Ca as described by Stephenson and Hensel (1982). Reported chemical trends resulting from seawater alteration of basalt (e.g., Hart, 1970) and spilitisation (Valiance, 1974) show enrichment in Na, K, Sr, Ba and Rb with loss of Ca and Si which, apart from Ca-depletion, are features shown by the Ivrea Zone samples. Some elements, such as Al, Ti, Zr, Y, and Nb generally remain relatively immobile (Pearce and Cann, 1973; Floyd and Winchester, 1975) except where the degree of alteration is very intensive (Valiance, 1974). As the extent of major element mobility in the Ivrea amphibolites is limited and concentrations of immobile incompatible elements closely match those of unaltered basalts, we can reasonably assume that the concentration of these immobile elements will reflect that of the parent basalt.
As previously alkali
basalt
described,
the amphibol~tes czm be ascribed to either tholeiitic or on the basis of their major element chemistry, hut as the
protoiiths
alkali content
has probably
be placed
on
differences
in trace element
this
(Pearce and Cann,
and
increased
during
it is more
instructive
abundances
1973; Langmuir
metamorphism,
not much reliance
to consider
and ratios between
the well established
basalts of different
et al., 1977; Pearce and Norry.
al., 1979). On the basis of Ti. P. Zr, Nb. Ba. Y and REE contents can be divided into two groups. First the type 1 amphiboiites, Strona
and at Nib&o
elements
(Table
exposed
in Val d’Ossoia
between
Forno
of Ba, are typical
of depleted
Type 2 amphiboiites,
rare-earth oceanic
et
the amphiboiites
and Sambughetto
in Vai of these
from I- 2 wt.%, Zr from 70 - 120 ppm. Nb
< 7 ppm, P,O, from 0.08 to 0.7 wt.%, Y from XI-50 ppm and Ba from 40-100 ‘They all have LREE-depleted
types
1979; Saunders
(Fig. 1). have quite low abundances
2; Fig. 5); TiO, ranging
can
patterns.
ppm.
These values. with the exception
tholeiites.
however, show a much greater range of values with Zr up to
250 ppm, Nb from 7.-33 ppm, TiO? up to 3.4 wt.%, P&I, up to 0.5 wt.% but Y is lower. only ranging up to 40 ppm. Ail samples in this group have LREE-enriched rare-earth
patterns,
One sample,
IV32, has very high Nb (56 ppm)
and
is more
alkaline than any other sample. There is good correlation between pairs of incompatible elements (Fig. 5) and an empirical division between alkaline and tholeiitic basalts
based
on Zr-P,OS
(Floyd
and
samples to have tholeiitic affinities. In Fig. 6, the amphibolites are plotted
(a)
(b)
Winchester.
1975) shows
on a Ti/lOO--Zr-Y
the majority
X 3 diagram
of
iPearce
,
Zr Fig. 6. (a) ‘G/100-22-Y plate basalts.
(b) Zr/Y-Zr
x 3 plot after Pearce
and Cann (1973). OFB-ocean
plot after Pearce and Norry (1979). Symbols
floor basal@,WPB-within as in Fig. 2.
199
and
Cann,
1973) and Zr/Y-Zr
accepted
that even modern
diagram,
the diagrams,
diagram
(Pearce
nonetheless,
provide
Figs. 6a and 6b, the type 1 amphibolites type 2 amphibolites no
tendency
and Norry,
1979). Although
day basalts do not always plot “correctly” some indication cluster
of basalt
in the ocean
type. In both
floor field while the
plot either in the ocean floor or the within plate fields. There is
in any
type
of discriminatory
talc-alkaline field. The type 2 amphibolites, elements, are more intimately
diagram
for samples
to plot
have
Nb
and
TiO,
levels
metasediments
in a
which have enhanced concentrations of incompatible mixed with the metasediments, hence it is necessary to
consider whether these higher levels are primary or the result of chemical with the surrounding metasediments. Many of the type 2 amphibolites, surrounding
it is
on this type of
higher
than
that
of the average
exchange however,
composition
(19 ppm and 1.2 wt.% respectively;
of the
Sills, unpublished
data), suggesting that the concentrations in the amphibolites are primary. In fact Figs. 2 and 3 suggest that it is the type 1 amphibolites which are the most altered. On
the basis
concluded the
type
of these
standard
discrimination
that the type 1 amphibolites 2 amphibolites
represent
diagrams
were depleted more
it can be tentatively
ocean-floor
incompatible
basalts,
whereas
element-enriched
oceanic
basalts and possibly even continental tholeiites. These inferences are further confirmed by examination of various trace element ratios (Table 4). Trace element ratios are more affected
useful
for comparison
by small amounts
of different
of fractionation
basalt
types
than absolute
because
ratios
abundances.
are less
One critical
ratio is Y/Nb which varies markedly between different basalt types (Floyd and Winchester, 1975). As can be seen from Table 4, the type 1 amphibolites, with an average depleted enriched
Y/Nb
ratio of 11.24 + 6.4 (2~) compares
(N-type) (E-type)
with that of a typical
MORB. The type 2 amphibolites are more comparable basalts. The same is true for almost all other element
Atlantic
with more ratios with
TABLE 4 Selected element ratios for the Ivrea Zone amphibolites (data from Tables 2 and 3) and analyses of various basalt types * Av. of
Av. of
Av. of
N-type
T-type
E-type
Alkali
type 1
type 2
378 + 379
MORB
MORB
MORB
basalt
Ba/Zr
0.6
1.0
1.7
Ba/Nb
14.8
8.1
Zr/Nb
25.8
Y/Nb
11.2
-
0.13
0.64
1.71
20.7
4
5.4
7.2
8.3
12.1
30
8.5
4.2
1.8
1.9
11
2.7
0.68
BCR-1
1.7
3.7
10
36
5
9.1
i 1
2.1
Zr/P@,
710
564
574
692
495
533
458
514
Ti/Zr
100
96
60
91
95
60
80
73
* N-type, E-type (enriched) and alkali basalt are taken from Wood et al. (1979a); the T-type (transitional) is from Wood et al. (1979b). data for BCR-1, a continental flood basalt is from Abbey (1980).
the exception
of those involving
been affected
by the metamorphism.
depleted
(N-type)
tholeiites,
MORB.
with more enriched compares
Figure 7 shows selected samples plots (not shown). when normalized can he compared
within
the FAMOUS
From the evidence between
a mid-ocean
to
to island-art:
reasonably
well with
tholeiites. whereas the type 3 dmphibolites c can hr: compared basalta. This wide range in basalt ~(~i~p~~sit~o~ls from Val Stmna
well with the range of composition.\
example
normaiised
for Sr and Nb.
the type 1 amphiholites
ocean-floor
cfepletecl
Similar
show large anomalies
In conclusion,
K. Rb. and Ba, the values for which }~avr pr(,bab]>
available
found in the present
region (Langmuir
day Atlantic,
for
et al.. 1977; Tame\;
et al.. 197%).
from these an~phib~~lites it is not possible
to disting~lish
ridge and back-arc
origin.
The type 1 amphibolites occur within a 2.5 km thick so it seems reasonable to attempt to relate them petrogenetically. Multi-ele-
(a) Tape 1 urnphiholites.
section
ment plots (Fig. 7’1demonstrate their close affinity with MORB in all but thr most lithophile elements. Their REE patterns (Fig. 8) show LREE depletion with Ce,/Yb, anomalies.
ranging from 0.66 to 0.98, and There is a range in REE abundances
they have very slight negative Eu with Ce, ranging from 9 to 17 times 7
Fig. 7. (a) Selected Attantic
N-type
type 1 amphibolites
and (b) selected
MORB (Wood et al., 1979a).
type 2 ampbibolites
normalised
to a typical
201 Type
5
1’ La Ce
1
Nd
f
Amphibolltes
I I
““1
Yb Lu
Sm Eu Gd Tb Dy Type
2
Amphibolltes
3
5u-.--kkLa Ce Pr Nd
Sm Eu Gd Tb Dy
Fig. 8. Chondrite normalised REE plots for representative type 1 and type 2 amphiboIites.
and Yb from
chondrite
the shape
of
the
decreasing
mg (sample
9 to 21 times chondrite
REE
patterns.
This
IV386 having
value of 49.1) and with increasing
in REE
an mg number
content
of a phase which has negligible
most
As shown
k,
in Fig. 4, the marked
change in
correlates
with
of 64.4 while IV201 has an mg
levels of other incompatible
fractionation likely.
but there is no significant
increase
elements.
for these elements, positive
correlation
This implies
olivine being the of Ni with mg
confirms the possibility of olivine fractionation. The negative Eu anomalies, coupled with the low Sr values (Fig. 7) suggests plagioclase has been removed. The relatively constant
ratios of Ti/Zr
(average
loo), Zr/Y
(2.5) and Zr/Nb
(26) suggest all these
elements remained incompatible during fractionation, eliminating clinopyroxene and hornblende as major fractionating phases. These data suggest that the most mg-rich samples,
such as IV386,
either
represent
a primary
liquid,
the remaining
liquids
developing by removal of olivine and possibly plagioclase, or that the most mg-rich samples contained cumulus olivine. As the rocks are completely metamorphosed and no igneous The range plagioclase (b) Type
textures remain, it is impossible to distinguish between these possibilities. of REE and high field strength elements is compatible with olivine and fractionation, which implies fractionation at relatively low pressures. 2 ~mp~ib~~i~es. As previously discussed the type 2 amphibolites appear to
have been less affected by low-temperature alteration but the range in composition is much greater. In view of this wide compositional range and the larger area over
which
the amphibolites
were sampled.
it is not realistic
(XRF data). C’e,,‘Yh,
The tectonic considerable
Of. ‘1Ht. .1MPHIROl.I I1.S
evolution
the origins
of the Ivrea Zone is p<)orly understood.
of the amphiholitts
bearing
to relate them
(Fig. Xt ranges from 3.6 tcl .?.I.
‘I b.C’TONIC‘ SIGNIFIC‘AN(‘1,
regarding
t
to the type 1 ~~t~ipllib~)lit~s have (‘~1, Y.. -- 1
p~trogel~~ti~al~y. All samples. in contrast
on this problem.
with the enclosing
There is no evidence
hence information metasediments
as to the nature
has a of anv
basement upon which a sedimentary sequence may have been deposited. Geophysical data suggest that rocks of mantle density approach the surface under the Ivrea Zone (Berckhemer, 1969). but there is no geophysical evidence for continental basement. The metasedimentt amphibolite zone may of course be allochthonous. It is also possible that the ~nterbandi~lg of ~lrnphib~~~ite and tlletased~ment is ;I primary feature of the processes (of crustal growth that may take place at a continental margin
during
development
of an accretionary
The metasediments are dominantly ranging from 52 to 70 wt.%, (Mehnart.
sedimentary
wedge.
composed of quartz-rich pelite with SO, 1975: Sills, unpublished data) and possibly
representing immature sediments such as greywacke (Mehnart. 1975 ). The atnphibolite-facies metasediments are extremely highly deformed so that all bedding has been completely
transposed
are numerous
and
and no original
have also been
bedding
transposed
features into
remain.
narrow
Small quartz
lenses
parallel
veins to the
foliation. The amphibolites. whatever their origin. have been similarly boudinaged into a series of lenses parallel with the regional foliation. The large Mafia-ultramafi~ complex (Fig. 1) intrudes the metasediments but is itself relatively undeformed. with igneous textures and cumulate structures locally preserved (Rivalenti et al.. 1975). The sediments are also intruded by Hercynian granitoids (Fig, 1). This means that the very intense deformation suffered by the metasediments and intercalated amph~boiites predates the emplacement of the mafic complex and is thus of Hercynian or earlier age. There is still uncertainty as to the age of the sediments and their metamorphism. Hunziker and Zingg (1980) obtained a Rh/Sr isochron of 475 of:20 Ma from 30 to 50 kg samples which they have interpreted as the age of granulite facies metamorphism and suggest sedimentation at 500- 700 Ma. However. zircon ages suggest metamorphism at a younger age. perhaps (K$@, 1974; Pm and Vielzeuf, 1983). although interpretation
as young as 300 Ma is subject to ambigu-
ity. The fate of sediments on the oceanic plate during subduction is of considerable interest. It is known that in some cases, such as the Mariana Island Arc. the sediments may be almost entirely subducted forming little or no accretionary wedge (Rarig and Kay, 1981). Even where there is an additional influx of continental-derived sediments, such as the Middle America Trench (Watkins et al., 1982) or the Japan trench (Von Huene et al., 1980), there may be more sediment subducted than accreted, provided that the continental sedimentation rate is relatively low. However,
203
where the sediment subducting
supply
from the continent
plate so that it approaches
are accreted
the trench
back on to the continental
1980). In southern
margin
Chile a major accretionary
late Palaeozoic-early and is exposed
Mesozoic (Forsythe,
on the trenchward
side of the present
made up of continent but is extremely
planes
dipping
at shallow
angles
towards
Moore
wedge developed
with minor
the
at a low angle and the sediments (cf. Alaska,
derived
deformed
and Allwardt,
in this way during
1982; Bartholomew
wedge is predominantly carbonates,
is higher, the effect is to depress
and Tarney.
Patagonian
Batholith.
greywacke-shale along
the continent.
the
in press). The
components
closely-spaced
All bedding
foliation
is completely
transformed and quartz veins, also transposed within the foliation. are abundant. The rocks are very similar to those in Val Strona. The Franciscan complex of California
(e.g., Ernst,
contains
1975). although
components
of oceanic
sediments. During
the formation
thickness
to consist
of an accretionary
predominantly
pre-existing
continental
ocean
or juxtaposed
crust
tectonic ocean
environment crust
basement.
wedge, it is possible
The accreted peridotitic
without
sediments mantle
quartz
stripped
grade, similarly
intermixed
with
immature
for the whole crustal the necessity
off and incorporated
plate.
the large
talc-alkaline
mafic
affinities
body
into
the accreting
deformation (Ernst, 1975; Karig, 1982). Low-grade and suffer dehydration, which leads to the develop-
veins in the overlying
in Val Sesia,
(Rivalenti
The fact that the amphibohtes MORB
and more enriched
on The
from the subducting
rocks.
Contemporaneous
kaline igneous activity may lie well to the continent side of the accreting Although there are no talc-alkaline volcanics in Val Strona. it is interesting that
of a
may be superimposed
of the overriding
is one where slivers of mafic volcanics
may be continually
of abundant
tectonically
of metasediment
against
wedge and subject to continual metasediments are unde~lated ment
much lower in metamorphic
affinity
which
intrudes
et al., 1975; Bigioggero in VaI Strona
tholeiite
or alkalic
wedge. to note has
et al., 19781979).
appear basaltic
the metasediments,
calc-al-
to represent compositions
both N-type is consistent
with an accretionary wedge model. The floor of most oceans is irregular due to the presence of seamounts, fracture zones and transform faults, many of which are the sites of alkali or enriched type MORB. It is these irreguIarities which are more Iikely to be stripped off and incorporated in an accretionary wedge during subduction. The rock type kinzigite may represent metasediment sive deformation in an accretionary wedge, followed
which has undergone pervaby high-grade post-deforma-
tional metamorphism as temperatures approached the equilibrium geotherm. Stronalite is simply the granulite facies equivalent. In Val Strona, stronahtes occur close to the mafic-ultramafic complex and the granulite facies metamo~hism is probably related to its emplacement (Schmid and Wood, 1976). This is further confirmed by the relatively low pressures of metamorphism of about 6-6.5 kbar (Newton and Haselton, 1981).
‘L‘ONC‘I.USIONS
(1)
Amphibolitrs
Zone
(2) On two
occurring
have igneous
rather
the basis
groups;
type
of their
ocean basalt
(3) The
(4)
occurrence
with
metasedinrentar>
formation
of
the
fvrra
they can he divided
into
prc~tolithx.
and cconpoaition,
with
strong
affinities
to depleted
a more varied composition
equivalent
N-type
MORB
to more enriched
compositions.
range in composition
by olivine
the
sedimentary
1 amphibnlites
and type 2 amphibolites (E-type)
w(thin
than
of the type 1 ~in~phih~~lit~s
suggests
It
fractj~~nat~~n. that this section of the crust was huiit
they are related
and FIagioclase is possible
wedge during
up ah an accretionary
the early to mid-Palaeozoic.
ACKNOWLEDGEMENTS
JDS gratefully acknowledges the receipt of an NERC post-doctoral research fellowship. We would like to thank Phil Potts and Olwen Thorpe at the Open University
for the INAA
We are very grateful
data and Nick Walsh at Kings College
for the assistance
of Giorgio
Rivalenti
for the ICP data.
in the field.
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