source: https://doi.org/10.7892/boris.115278 | downloaded: 9.8.2022
Progressive
Low-Grade Metamorphism of a Black
Shale
Formation, Central Swiss
Alps,
with Special
Reference
to Pyrophyllite and Margarite Bearing
Assemblages*
^MARTIN FREYf
Mineralogisch-petrographisches Institut der Universitdt Bern, Sahlistrasse 6, CH-3012 Bern,
Switzerland
(Received 23 July
1976,
in
revised form
21 February 1977)
ABSTRACT
The unmetamorphosed equivalents
of
the regionally metamorphosed clays and marls that make
up the Alpine Liassic black shale formation consist
of
illite, irregular mixed-layer illite/montmoril-
lonite, chlorite, kaolinite, quartz, calcite,
and
dolomite, with accessory feldspars
and
organic
material.
At
higher grade,
in
the anchizonal slates, pyrophyllite
is
present and
is
thought
to
have
formed
at
the expense of kaolinite; paragonite and
a
mixed-layer paragonite/muscovite presumably
formed from the mixed-layer illite/montmorillonite. Anchimetamorphic illite is poorer in Fe and Mg
than
at the
diagenetic stage, having lost these elements during
the
formation
of
chlorite. Detrital
feldspar has disappeared.
In epimetamorphic phyllites, chloritoid
and
margarite appear
by the
reactions pyrophyllite
+
chlorite
=
chloritoid
+
quartz
+ H
2
O and
pyrophyllite
+
calcite
±
paragonite
=
margarite
+
quartz
+
H
2
O
+
CO
2
, respectively. At the epi—mesozone transition, paragonite and chloritoid seem
to become incompatible in the presence of carbonates and yield the following breakdown products:
plagioclase, margarite, clinozoisite (and minor zoisite),
and
biotite. The maximum distribution
of
margarite
is at the
epizone-mesozone boundary;
at
higher metamorphic grade margarite
is
consumed by
a
continuous reaction producing plagioclase.
Most of the observed assemblages in the anchi- and epizone can be treated in the two subsystems
MgO (or FeO)-Na
2
O-CaO-Al
2
O3-<KAl3O5-Si0
2
-H
2
O-CO
2
). Chemographic analyses show that
the variance of assemblages decreases with increasing metamorphic grade.
Physical conditions are estimated from calibrated mineral reactions and other petrographic data.
The composition
of
the fluid phase was low
in X
c(h
throughout the metamorphic profile, whereas
X
CIU
was
very high, particularly
in the
anchizone where
a
H
was probably
as
low
as
0-2. P—T
conditions along the metamorphic profile are
1-2
kb/20O-30O
°C in
the anchizone (Glarus Alps),
and
5
kb/5OO-550
°C at the
epi-mesozone transition (Lukmanier area). Calculated geothermal
gradients decrease from
50
°C/km
in the
anchimetamorphic Glarus Alps
to 30
°C/km
at the
epi-mesozone transition of the Lukmanier area.
INTRODUCTION
THE
central Swiss Alps offer
the
rare opportunity
to
study specific
lit-
hostratigraphic
units
all the
way from unmetamorphosed sediments
to
medium-
grade
metamorphic
rocks.
One such study,
on a
Triassic red bed formation
has
•
This paper is
a
condensed version of
the
author's 'Habilitationsschrift'.
f
Present address: Mineralogisch-petrographisches Institut der Universitat Basel, Bernoullistrasse 30, CH-
4056
Basel, Switzerland.
IJourn*!
of
Petrology, Vol. 19, P«rt 1, pp.
95-135,
19781
96 M. FREY
already been documented (Frey, 1969a). The purpose of the present paper is to
present complementary data on a related sequence of Liassic black shales.
Regional setting
The Liassic black shale formation studied was found at several localities: in the
Jura mountains (the unmetamorphosed region), in the boreholes underneath the
Molasse Basin and in the Helvetic Zone, and at the northern boundary of the
Lepontine region (area of highest grade, see Fig. 1).
The unmetamorphosed clays and marls are exposed in the Tabular Jura of
southern Germany and northwestern Switzerland, where they form part of the flat-
lying foreland of the Alpine orogeny. The southern extension of the Tabular and
Folded Jura has been found in several boreholes below the Molasse Basin to a
depth of about 2 km (Buchi et ai, 1965).
FIG.
1. Simplified geologic map of the central Swiss Alps showing main sample localities (black dots). Stippled
areas:
Jura and Helvetic domains. Metamorphic mineral zone boundaries after Niggli & Niggli (1965) with
minor modifications. Abbreviations for localities: PP = Panixerpass; VC = Valle CavaJasca; VP = Val Piora.
METAMORPHISM OF BLACK SHALES, SWISS ALPS 97
The anchimetamorphic shales and slates of this formation (the anchizone being
defined on the basis of illite crystallinity—Kiibler, 1967) are found in the Helvetic
Nappes, which form the Alpine border region of central Switzerland. The Helvetic
Nappes consist mainly of miogeosynclinal sediments which slid northwards off the
Aar and Gotthard Massifs in great recumbent folds and thrust slices (Triimpy,
1960).
Until recently, the Helvetic Nappes were considered unmetamorphosed.
The epi- and mesometamorphic phyllites and schists belong to the northern and
southern sedimentary cover of the Gotthard
Massif,
respectively. The Gotthard
Massif itself lies within the chloritoid zone of Alpine metamorphism as defined by
Niggli&Niggli(1965).
Stratigraphy
The Liassic rocks of the Jura mountains consist of 20-60 m of grey to black
clays,
marls, sandstones and arenaceous limestones (Heim, 1919). In the Glarus
Alps,
these rocks reach a thickness of 350-500 m (Triimpy, 1949). At its base, the
Liassic sequence is composed of 30 m of sandstones with intercalated black shales
and slates (= member I in Fig. 5). Overlying rock units (= member II and III in
Fig. 5) consist of 100-150 m of grey to black marly shales and slates with
intercalations of arenaceous limestones. The rocks which in turn overlie these
marly shales and slates are mainly arenaceous limestones and were excluded from
this study. Further south, in the Urseren Zone, the Liassic rocks consist of 50-70
m of black to grey phyllites and schists with intercalated arenaceous limestones at
the base (= members I?, II and III in Fig. 7), and 40-80 m of arenaceous
limestones at the top (= member IV in Fig. 7—see Niggli, 1944). At the Lukmanier
Pass these Liassic sediments are at least 700 m thick and consist of a basal 10 m of
pelitic black schists with minor quartzites (= members I and II in Fig. 9), overlain
by 50 m of alternating marly black schists and limestones (= member III in Fig. 9).
Only the lower Liassic of the Lukmanier area has low-grade equivalents in the
Urseren Zone and the Glarus Alps (Baumer et al.,
1961;
Frey, 1967).
METHODS
Sampling procedure. Whenever possible, complete stratigraphic sections were
sampled taking into account all lithological variations. At most localities 10 to 25
specimens were collected at intervals of decimeters to several meters.
Mineral identification. For the unmetamorphosed and metamorphosed sedi-
ments of the anchizone, mineral determinations were obtained primarily by X-ray
studies. In all other cases, combinations of optical and X-ray methods were
applied. The identification of the sheet silicates, including the clay minerals, was
made by X-ray diffractometer and Guinier camera techniques. The X-ray mounts
were prepared by sedimentation, and were air-dried, glycolated or heat treated
where necessary. Accurate determinations of rf-spacings were obtained using Si or
quartz as internal standards.
Modal analysis. Quantitative determination of quartz and feldspars was done by
X-ray analysis, using a method devised by Peters (1965, 1970). The amounts of the
various carbonate minerals were calculated from the known calcite/dolomite ratio
98
M. FREY
coupled with volumetrically determined CO
2
values in the bulk samples. Sheet
silicate abundance could only be determined semiquantitatively by X-ray
diffractometry; the method used was that outlined by Henderson (1971). In all
cases,
the X-ray standards used were prepared by mechanically mixing equal
amounts of various combinations of minerals, followed by measurements of the
intensity ratios of the reflexions indicated in Table 1.
TABLE 1
Intensity-ratios used for semiquantitative determination of sheet silicates. The ratios
shown result from equal proportions of the minerals listed
Mineral
Hike
Hike
Illite
Chlorite
Muscovite
Muscovite
Muscovite
Muscovite
Muscovite
Muscovite
Margarite
Muscovite
Muscovite
A
hkl
00
1
001
00
1
004
002
006
006
002
006
00.
10
00.
10
060
060
dinA
10
10
10
3-56
10
3-3
3-3
10
3-3
20
20
1-50
1-50
Mineral
Montmorillonite
Chlorite
Kaolinite
Kaolinite
Pyrophyllite
Paragonhe
Paragonite/muscovhe
Margarite
Margarite
Margarite
Paragonite
Paragonite
Margarite
B
hkl
00 1
002
002
002
001
006
—
002
006
00. 10
00. 10
060
060
dinA
17
7
3-60
3-60
9-2
3-20
3-25
9-6
3-2
1-91
1-92
1-48
1-47
A:B
Intensity ratio
1:3
1:2
1:2
1:1
1:1
1:1
1:1
7:1
1:1
5:4
4:5
1:1
1:1
Chemical and microprobe analyses. Whole rock analyses, were done by standard
wet chemical methods. Some of the mineral analyses were performed at Cambridge
University by J. S. Fox on a Geoscan electron-probe microanalyser, using simple
silicates and oxides as standards. Corrections were done according to the
procedure described by Sweatman & Long (1969). The remaining analyses were
carried out by the author on an Acton—Cameca electron-probe at Yale University
using standards with a composition close to that of the unknown mineral. In this
case,
corrections were made with the method of Bence & Albee (1968).
Where minerals were too fine-grained to be analysed satisfactorily by
microprobe, their chemical compositions were estimated from X-ray data.
MINERAL COMPOSITIONS AND ABBREVIATIONS
The compositions of the phases based on which the stoichiometry of the
reactions were calculated and the abbreviations for mineral names used in the
tables and figures are listed below:
ab = albite,
cc = calcite,
chl = chlorite,
NaAlSi
3
O
8
CaCO
3
(Fe,Mg)
4
.
5
Al
3
Si
2
.
5
O
10
(OH)
8
METAMORPHISM
OF
BLACK SHALES, SWISS ALPS
99
ctd
dol
ma
pa
py
zo
bi
clz
c/m
gr
i
i/m
kaol
= chloritoid,
= dolomite,
= margarite,
= paragonite,
= pyrophyllite,
= zoisite,
= biotite
= clinozoisite
FeAl
2
SiO
5
(OH)
2
CaMg(CO3)
2
CaAl
4
Si
2
O
10
(OH)
2
NaAl
3
Si
3
O
10
(OH)
2
Al
2
Si
4
O
10
(OH)
2
Ca
2
Al
3
Si
3
0
12
(0H)
kf
ky
= chlorite/ montmorillonite
mu
mixed-layer
= garnet
= illite
pa/mu
plag
= illite/montmorillonite rect
mixed-layer
= kaolinite
st
= K-feldspar
= kyanite
= muscovite
= paragonite/muscovite
mixed-layer
= plagioclase
= rectorite
= staurolite
BULK ROCK CHEMISTRY
To test for isochemistry in the formation, composite rock samples were analysed
from all the four areas under consideration. As can be seen from Table 2, the mean
rock compositions are similar in all four areas, although the chemical diversity in
a
single outcrop may
be
considerable (see below) with the largest variations due
to
the changing carbonate content.
MINERALOGY AND PETROGRAPHY
The unmetamorphosed marls and claystones of the Tabular Jura and the boreholes
under the Molasse Basin
The unmetamorphosed Liassic rocks from under the Molasse Basin were studied
using drill hole samples from the Berlingen, Kreuzlingen and Lindau localities (see
Fig. 1). Additional data by Peters (1964) was available from the Frick area. In all,
a total of 34 samples were investigated.
Mineralogy
Illite and irregular mixed-layer illite/montmorillonite. Illite was determined
by
its strong basal reflexions
at
10 A and 5 A. Diffractograms of the air-dried mixed-
layer illite/montmorillonite showed
a
broad basal reflexion
at
about 10-13 A. On
glyeolation, this reflexion shifted
to
roughly 12-14
A,
indicating the presence
of
20-^tO per cent expandable layers (MacEwan et al., 1961,
fig.
XI.
17).
Absence of a
regular sequence
of
higher and lower order basal reflexions indicated
a
random
interstratification
of
illite
and
montmorillonite.
The
(060)-reflexion varied from
1-498
to
1-504 A, indicating up to 25 per cent Mg and Fe in the octahedral layer
(Maxwell & Hower, 1967,
fig.
4).
Chlorite. The first five basal reflexions appeared on X-ray diffractograms.
In
six