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OCR for page 96
Hydrothennal Systems Associated with
Regional Metamorphism and
~, . .
6
~;rusta~ Anatexis:
Pyrenees, France
.
STEPHEN M. WICKHAM
University of Chicago
HUGH P. TAYLOR, JR.
California Institute of Technology
INTRODUCTION
Our understanding of the transport of fluids through the
deeper parts of the crust, and in particular through rocks
~ . . ,
undergoing prograde regional metamorphism, is at an
embryonic stage. This stems largely from the difficulty of
making direct observations of such processes, in contrast
to the situation in active shallow hydrothermal systems
(which are accessible to study in boreholes). Stable iso-
tope (~8O/160, 13C/~2C, and D/H) studies of metamorphic
and igneous rocks and minerals may, however, be used to
place constraints on the passage of H2O- and CO2-rich
fluids through the crust. Oxygen, hydrogen, and carbon
are major constituents of both rocks and typical crustal
fluids, and these three elements show systematic differ-
ences in isotopic composition in the different terrestrial
reservoirs.
For example, the mantle has a ALSO value of about +6,
distinctly different from the range of values shown by
most detrital sedimentary rocks (+9 to +18) and much
lower than the values in sedimentary carbonates (+20 to
+30~. All of these "normal" rock INTO values are distinctly
higher than the values in seawater (EGO = 0) or meteoric
water (BOO =-25 to 0) (Taylor, 1974, 1977~. Conse-
quently, if aqueous fluids derived from the Earth's surface
interact with igneous or metamorphic rocks at elevated
96
Examples from the
temperatures where the equilibrium isotopic fractionations
are small, this results in a change in the rock oxygen (and
hydrogen) isotopic composition toward that of the water.
Stable isotope analyses of such rocks can help to identify
the source of infiltrating fluids, and, in the case of oxygen,
which is the dominant constituent of H2O, CO2, and virtu-
ally all crustal rocks, these data can be used to make
material-balance calculations constraining the quantity of
fluid involved.
In a number of areas it has been recognized that whole-
rock INTO values tend to decrease with increasing meta-
morphic grade (e.g., Garlick~and Epstein, 1965; S-hieh and
Taylor, 1969; Shieh and Schwarcz, 1974~. For instance, in
Idaho the variation is from about +15 in low-grade shales
to +11 in sillimanite-grade rocks formed during Creta-
ceous metamorphism (Fleck and Criss, 1985~. There is,
however, at present no clear consensus of opinion regard-
ing the origin of this isotopic shift, its timing, or its rela-
tionship to fluid transport during metamorphism. For
example, the variation of 6~80 from typical sedimentary
values to values closer to typical mantle values could
involve minimal amounts of fluid and arise simply by
exchange and homogenization of oxygen between high-
~8O metasediments and low-~8O mantle-derived igneous
lithologies. Such an effect might be expected at increasing
OCR for page 97
HYDROTHERMAL SYSTEMS ASSOCIATED WITH REGIONAL METAMORPHISM AND CRUSTAL ANA TEXIS
depth within the crust, where mantle-derived material
becomes progressively more common and sedimentary
material more rare. Such exchange conceivably could
take place at a relatively low fluid-rock ratio with the fluid
merely acting as the agent of isotopic exchange.
On the other hand, the shift in 6~80 to lower values at
higher metamorphic grade could reflect exchange of oxy-
gen between higher-grade metamorphic rocks and low-~8O
fluids at high temperatures, with the fluid itself forming
the isotonically light oxygen reservoir. In this case large
quantities of fluid would be required to bring about bulk
changes in INTO by the amounts that are commonly ob-
served (3 to 4 per mill. If the latter process is dominant,
then permeabilities must be appreciable and fluid-rich
conditions are likely to be common during metamorphism
(e.g., Ferry, 1986), with fluid pressure at times equaling or
locally exceeding lithostatic pressure. If the former inter-
pretation is more common, a free fluid phase may only be
intermittently and/or locally present.
The Hercynian metamorphic terrane exposed in the
Pyrenees offers a good opportunity to differentiate be-
tween these two alternative scenarios. Here, a characteris-
tic shift to lower-~8O values with increasing metamorphic
grade is clearly observed, and good exposure, clear-cut
geological relationships and access to a wide range of
structural levels allow us to place tight constraints on both
the nature and scale of fluid-rock interaction during meta-
morphism (Wickham end Taylor, 1985, 1987; Wickham,
1987a; Bickle et al., 1988~.
France
[RO/S SE/G~RS
'-~ ~XSS/F
~7 St. Girons ,~,/ If
=~n~:
'art
Spa i n
Mesozoic and Tertiory sediments
~3 Low-grode Paleozoic
High-grode mice schists, migmalites,
anotectic reroutes
~3 Orthogneisses
Basal gneisses
~3 Lote grenodior~tes
SOKM
, ,
97
METAMORPHISM IN THE PYRENEES
Hercynian Basement of the Pyrenees
The Pyrenees are a roughly linear chain of mountains
between France and Spain. They were uplifted during the
lower Tertiary in response to convergence between Iberia
and Western Europe. Uplift has exposed an extensive pre-
Mesozoic basement terrane (the Hercynian basement)
comprising Paleozoic and Precambrian sediments,
metasediments, and gneisses as well as a variety of grani-
toid plutons (see Figure 6.1~. All of these lithologies were
metamorphosed or intruded at about 310 to 340 Ma during
the Late Carboniferous, Hercynian orogeny (Zwart, 1979;
Autran et al., 1980; Bard et al., 1980; Bickle et al., 1988~.
Subsequent (e.g., Tertiary) deformation and recrystalliza-
tion of the Hercynian metamorphic and granitic rocks were
relatively minor.
The Hercynian basement ranges from virtually unmeta-
morphosed, fossiliferous Devonian and Carboniferous
rocks, some of which were actually being deposited while
metamorphism was occurring at depth, through Lower
Paleozoic mica schists, marbles, and migmatites, finally to
amphibolite and granulite-facies "basal gneisses" (Zwart,
1979~. The mineral assemblages in the gneisses formed at
pressures of 4 to 7 kbar (Vielzeuf, 1984), compatible with
their being the basement upon which the Paleozoic sedi-
mentary sequence was deposited (Vitrac-Michard and
Allegre, 1975~. The metamorphic sequences are charac
FIGURE 6.1 Hercynian basement outcrop
in the Pyrenees, showing the various lo-
calities discussed in the text.
OCR for page 98
98
terized by very abrupt transitions from the low- to the
high-grade regions and by extensive partial melting of
Lower Paleozoic politic metasediments at temperatures of
about 700°C and depths of only about 10 to 12 km. This is
recorded by abundant politic migmatites and peraluminous
granitoids associated with the high-grade mica schists.
Subdivision of the Hercynian Crust into Three
Structural Levels
Taken together the wide range of structural levels ex-
posed in the different tectonic blocks of the Hercynian
basement of the Pyrenees can be interpreted in terms of a
composite section through the upper 25 km of the conti-
nental crust. Because the oxygen isotope systematics are
very distinctive at different structural levels within this
section, it is convenient to divide it into three tectonostra-
tigraphic zones: (1) a moderately deformed, fossiliferous
sedimentary sequence that was deposited from the Upper
Ordovician to the Upper Carboniferous; (2) a metamor-
phic and migmatitic sequence developed within Cambro-
Ordovician (mainly politic) metasediments, with equili-
bration pressures of 2 to 4 kbar, that were locally exten-
sively melted and intruded by peraluminous leucogranites
and biotite-cordierite granites; and (3) a region of amphi-
bolite- and granulite-facies orthogneisses and paragneisses
equilibrated at 4 to 7 kbar.
Although a continuous section through all of these crustal
levels is not exposed in the Pyrenees, rocks from all three
levels outcrop within several relatively restricted geographic
areas (e.g., in the Agly and St. Barthelemy massifs). Figure
6.2 shows a section through the St. Barthelemy Massif,
indicating the relationship between Zones 1, 2, and 3 in
that region.
FIGURE 6.2 Schematic ~ue-scale section
through the St. Barthelemy Massif redrawn
from Passchier (1984, Figure 3~. The line
of section is approximately north to south
(from low to high grade) and is about 15
km long. The Zone 3 "basal gneisses"
have uncertain structural relationship to
the other rocks in the massif because they
are bounded by a mylonitic shear zone
that approximately separates Zone 2 and
Zone 3.
STEPHEN M. WICKHAM AND HUGH P. TAYLOR, JR.
Metamorphism and Anatexis
The very abrupt, albeit progressive and gradational,
transition from low to high grade in the metamorphic
sequences in the Pyrenees implies the existence of very
steep thermal gradients during metamorphism. Typically,
a progression in the Zone 2 politic rocks (which comprise
over 90 percent of the Lower Paleozoic metasediments)
from chlorite-sericite phyllites through andalusite an :l sil-
limanite schists to migmatites and biotite-cordierite grani-
toids occurs over horizontal distances of only 3 to 5 km
(e.g., in the Trois Seigneurs Massif, Figure 6.3~. Thin (10
to 50 m) carbonate-rich beds are interlayered within the
Cambro-Ordovician pelites; these have developed various
calc-silicate mineralogies consistent with the metamor-
phic mineral assemblages in the adjacent pelites.
The granitic rocks were mostly derived by partial melt-
ing and homogenization of the high-grade politic metasedi-
ments, and in general the granitic melts have not migrated
very far from their region of generation (Wickham, 1987a).
Migmatite textures and compositions imply that pelites
melted to at least 50 to 60 percent by volume, and the
observed rapid increase to these high degrees of melting at
relatively low temperatures of 700° to 750°C strongly
implies water-rich conditions with aH2O buffered exter-
nally to high values (see Wickham, 1987a). This is also
suggested by common pegmatitic textures, widespread
muscovitization and tourmalinization, and by calc-silicate
mineral assemblages in the marble layers (they commonly
contain clinozoisite and sometimes also grossular; see Ferry,
1983~.
In contrast to the Zone 2 rocks, much smaller degrees
of melting occurred within the amphibolite- and granulite-
facies "basal gneisses" (Zone 3~. These rocks are litho-
logically distinct from Zone 2 in that they contain far less
ZONE 3 - ~ZONE 2 ~ -'a ZONE 1
(4-7kbar) ~(2-4 fiber) , (<2kbar)
i;
!
(2-4 kbar)
mylonite
z°ne~l~minmatites ~mien Hitch ~choler ASH_
1
omphibolite and granulite
~ . .
racier gne~sses
OCR for page 99
HYDROTHERMAL SYSTEMS ASSOCIATED WITH REGIONAL METAMORPHISM AND CRUSTAL ANA TEXIS
SW
I ,*
2000
E
1000
Carbonate //,
\ ~
it''
+'%,~~E,~
Florid I
* ~x ~ / I ~
Chlorite sericite phyllit"
Carbonate
E23 Andalusite schist
Id l Andalusitc + sillimanite schist
_~
Biotite - sillimanite schist
Betide - sillimarute gneiss
E;2l Biotite granite - quartz diorite
{i:3 Leucogranite
migmatite
cone
petite and larger amounts of granitic orthogneiss, quartzo-
feldspathic biotite gneiss, and amphibolite. Zone 3 is
locally migmatitic but contains virtually no large (>100 to
1000 m) bodies of peraluminous granite. Vielzeuf (1984)
made a detailed study of the petrology of these rocks,
which are commonly orthopyroxene bearing and clearly
equilibrated at greater depth (4 to 7 kbar) and under much
"dryer" peak metamorphic conditions than those in Zone
2. Although no H2O activity data are available from this
work, some of the granulites probably equilibrated at aH2O
> 0.5. This does not in itself preclude the possibility that
more water-rich conditions prevailed at an earlier stage
during the metamorphism. However, as described below,
the stable isotope systematics within the basal gneiss Zone
3 rocks are significantly different from those in the mica
schist-migmatite sequences (Zone 2~. This almost cer-
tainly reflects contrasting fluid-rock interactions during
metamorphism at these different levels within the Her-
cynian crust.
OXYGEN ISOTOPE SYSTEMATICS
Zone 1
The Paleozoic rocks from Zone 1 comprise fossilifer-
ous shales and carbonates that have been deformed but
only very weakly metamorphosed. Rocks are typically
fine "rained and may contain chlorite or fine-grained white
mica but no higher-grade metamorphic minerals. Because
they probably never experienced temperatures in excess of
400°C during the Hercynian metamorphism, it is hardly
surprising that these rocks in the main preserve typical
99
FIGURE 6.3 Schematic true-scale section
through the Trois Seigneurs metamorphic
sequence, indicating the progressive gra-
dation from low-grade phyllites through
high-grade mica schists and migmatites to
the granitoids that were derived mainly
through anatexis of the politic metasedi-
ments. Note the proximity of the low- and
high-grade rocks.
sedimentary INTO values. However, recent 87Sr/86Sr data
(Bickle et al., 1988) indicate that even these low-grade
phyllites and shales were extensively modified by fluid
infiltration during prograde metamorphism. 87Sr/86Sr ratios
were homogenized (and in some cases lowered) to values
of ~0.715 at 310 Ma, a process that is observed in even the
lowest-grade Zone 1 shales, which could not have experi-
enced temperatures any higher than 250° to 300°C (see
Table 6.1~. i80/~60 ratios may have been modified at the
same time, but we have no way of detecting this because
we cannot determine the depositional INTO values of these
sediments (as we can with 87Sr/86Sr). The Sr data allow us
to infer that significant isotopic modifications resulting
from fluid infiltration commenced at the earliest stages of
prograde metamorphism and continued up to peak meta-
morphic temperatures (see Bickle et al., 1988, and below).
The ~8o/~6o data are illustrated in Figure 6.4, where a
compilation of INTO values for Zone 1 (calcite for the
carbonate rocks and whole rock for shale samples) is pre-
sented from four different parts of the Pyrenees. In the
Trois Seigneurs and Agly massifs these rocks are Ordovi-
cian and Silurian shales, whereas in the St. Barthelemy
and Arize massifs they are Ordovician to Carboniferous
shales and limestones. Calcite from the limestones ranges
from +19 to +24, consistent with the typical range of
isotopic values shown by many Paleozoic sedimentary
carbonates (Baertschi, 1957; Veizer and Hoers, 1976~.
Shales range from +13 to +16, with most values between
+14 and +15. These kinds of values are also shown by all
other Zone 1 Ordovician to Carboniferous sedimentary
rocks that we have analyzed from other parts of the Pyre-
nees.
OCR for page 100
100
STEPHEN M. WICKHAM AND HUGH P. TAYLOR, JR.
TABLE 6.1 Oxygen and Strontium Isotope Data (average values) at Various Outcrop Localities of Pelitic Schist, Phyllite,
and Shale in the Trois Seigneurs Massif, Arranged into Four Groups as a Function of Grade of Metamorphism
Original Sedimentary Intermediate Grade High grade
Parameter Rock (model values Low Grade (shales (vicinity of the (andalusite
Studied 450 Ma)a and phyllites) biotite isograd) sillimanite zones)
Mean 6~80 value
Range of 6~80
Mean 87Sr/86Sr value
Range of 87Sr/36Sr
Average Sr content
Average Rb content
Average Rb/Sr ratio
?
0.7202 + 0.0053 (5)
0.7133 to 0.7326
89 ppm (5)
138 ppm (5)
1.55
+14.5 + 0.7 (8)
+13.3 to +16.0
0.7130 + 0.0022 (5)
0.7089 to 0.7166
89 ppm (5)
138 ppm (5)
1.55
+12.1 + 0.9 (5)
+10.5 to +13.3
0.7153 (1)
102 ppm (1)
98 ppm (l)
0.96
+11.5 + 0.6 (16)
+10.6b to +12.7
0.7151 +0.0016 (6)
0.7129 to 0.7178
98 ppm (7)
160 ppm (7)
1.62
Note: Data are from Bickle et al. (1988) and Wickham and Taylor (1985). The tabulated 87Sr/86Sr, Sr, and Rb data represent the mean
values for the various localities, using a single average value for each of these outcrop localities where multiple samples were studied.
The + indicates average deviation from the mean value, with the number of analyzed localities given in parentheses. Al187Sr/86Sr values
are calculated at 310 Ma except for the original sedimentary rock (model) values, which are calculated for 450 Ma.
aThe 6~80 values are unknown, but the model strontium isotope values at 450 Ma can be calculated, as discussed in the text.
bOne sample with an anomalously low-0 value of +8.8 is not included (Wickham and Taylor, 1985).
Zone 2
In contrast to the characteristic sedimentary INTO values
shown by the shales and carbonates of Zone 1, the Zone 2
mica schists, migmatites, and peraluminous granitoids have
lower and much more uniform GINO values, mostly in the
range +11 to +12 (see Table 6.1 and Figure 6.4~. This is
particularly clear in the Trois Seigneurs Massif, where the
Zone 1 to Zone 2 transition is well exposed and where the
Zone 2 carbonates have internally homogeneous GINO values
that are essentially identical to the INTO of the adjacent
mica schists (Wickham and Taylor, 1985~. A detailed
isotopic profile through one of the Trois Seigneurs meta-
carbonate layers is shown in Figure 6.5. This unit lies
between the "sillimanite in" isograd and the migmatite
zone and is about 15 m thick, sandwiched between psam-
mitic rocks with INTO values of about +12. The calcite has
a fairly uniform GINO of between +13 and +14 throughout
the layer, regardless of the calcite content of the sample or
the distance of the sample from the margin of the layer.
This indicates an extremely high degree of oxygen iso-
topic equilibration between this layer and the surrounding
metasediments, in contrast to the situation prior to meta-
morphism when the carbonate layers would have had much
higher GINO values than the surrounding rocks (as is still
the case in Zone 1~. This homogeneity of carbonate and
petite oxygen isotopic compositions within the Trois
Seigneurs Zone 2 rocks is typical of the oxygen isotope
systematics at this structural level throughout the Pyre-
nees. Preexisting sedimentary heterogeneities in GINO have
everywhere been smoothed out by pervasive oxygen iso
topic exchange over wide regions. The Hercynian age of
this process is proved by Rb-Sr isochrons obtained from
suites of mica-schist samples (Bickle et al., 1988) from
Zone 2, indicating homogenization of 87Sr/86Sr at values of
~0.715 at 310 Ma. Interestingly, in the case of Sr the 87Sr/
86Sr ratios are not smoothed out everywhere; this is proba-
bly because the giant reservoir of strontium in the carbon-
ates (containing ~2000 ppm Sr) is much more resistant to
change than the petite reservoir (~200 ppm Sr) and thus
tends to retain its original sedimentary value of~0.708
(Bickle et al., 1988~.
In addition to the whole-rock GINO values being much
more homogeneous in Zone 2 than in Zone 1, there is an
obvious, pronounced lowering of 6~80 going from Zone 1
to Zone 2 (Figure 6.4; Table 6.1~. This shift reflects a
change in the bulk oxygen isotopic composition of the
Paleozoic sedimentary pile of 3 to 4 per mil, from about
+15 to about +11.5 (similar to that observed in some other
regional metamorphic sequences (e.g., Garlick and Ep-
stein, 1967; Rye et al., 1974; Fleck and Criss, 1985). This
shift is particularly well characterized in the Hercynian of
the Pyrenees, where it takes place fairly abruptly over a
stratigraphic distance of 1 or 2 km within the metamorphic
sequence.
It is clear that the Zone 1 and Zone 2 politic rocks both
belong to the same Paleozoic sedimentary succession and
that they originally had similar bulk isotopic and chemical
compositions. This implies that the observed shift in GINO
values does not represent any lithological difference be-
tween Zone 1 and Zone 2. During metamorphism these
rocks were partially dehydrated and decarbonated, con
OCR for page 101
HYDROTHERMAL SYSTEMS ASSOCIATED WITH REGIONAL METAMORPHISM AND CRUSTAL ANA TEXIS
versed to high-grade mica schists and marbles, were iso-
topically homogenized, and overall had their bulk 6~80
values lowered by about 3 to 4 per milt The Zone 2
metamorphic rocks clearly must have exchanged oxygen
with a large reservoir that had a 6~80 value significantly
lighter than +1 1.
Zone 3
Exposures of Zone 3 rocks in the Pyrenees are limited
and principally occur in three regions: the Agly, St. Barthe-
lemy, and Castillon massifs (see Figure 6.1~. Addition-
ally, a small area near the village of Lapege in the Trois
Seigneurs Massif exposes rocks that probably correlate
with the basal gneisses elsewhere because they are litho-
logically distinct from any other rocks in the Trois Seigneurs
area (which clearly belong to Zone 2; see Wickham, 1987a).
Zone 3 rocks comprise a lithologically heterogeneous
sequence of granitic gneisses, quartzo-feldspathic biotite
gneisses, amphibolites, carbonates, and relatively rare
+24 _
+2`
+~E
~0
ZONE 1 I ZONE 2
l
O carbonates
· pastes
by\
_ ~St. Barthelemy
.~ ~ ~--e'0:
+10 _
+8
Trois Saigneurs
PALEOZOIC SEDIMENTS METAMORPHOSED PALEOZOIC
(SHALES AND CARBONATES) (MICA SCHISTS AND MARBLES)
FIGURE 6.4 Compilation of data for Zones 1 and 2 from the
Agly, Arize, St. Barthelemy, and Trois Seigneurs massifs for
whole-rock petite samples (solid circles) and calcite from car-
bonate-bearing samples (open diamonds). The data are plotted
as a function of metamorphic grade with their position on the
diagram corresponding roughly to their relative distance from
the Zone 1 to Zone 2 boundary in the field. All of the Zone 2
carbonate samples are from Trois Seigneurs, and the two data
points with error bars represent the average values (and the 6~80
range) for the two detailed profiles in Figures 6 and 7 of Wick-
ham and Taylor (1987~. There is a pronounced shift in SILO to
lower values going from Zone 1 to Zone 2, and the Zone 2 values
are much more homogeneous than those in Zone 1 (also see
Table 6.1~.
101
TROI S SEIGNEURS
/
~0(1%)
~ CARBONATE
12 _
-
-
~v 10
-
8
He
~ 6
In
0~ (61%)
0 ~0 (31 %)
o~hO(72%)
0~0 (77%)
/
+10 +12 +14 +16 +18
8180
o
FIGURE 6.5 Oxygen isotopic profile through a carbonate unit in
the Trois Seigneurs Massif (sample SI13, Wickham and Taylor,
1985) that crops out between the "sillimanite in" and "andalusite
out" isograds (see Figure 6.3~. Note that the INTO of calcite is
fairly uniform throughout the layer, regardless of either the dis-
tance from the margin or the calcite content of the sample. The
INTO values within the carbonate layer are similar to the psam-
mites to either side, and they are much lower than the original
sedimentary values, which must have been at least as high as +22
to +25.
kinzigites (politic gneisses). Oxygen-isotope data from
these regions are summarized in Figures 6.6 and 6.7 (see
also Wickham and Taylor, 1987~. These data are not as
clear cut as those obtained from Zones 1 and 2, mainly
because we do not know for certain what the original
premetamorphic INTO values of the Zone 3 lithologies were;
we simply do not have access to the unmetamorphosed
equivalents of these rocks (as we do in Zone 2~.
The data in Figures 6.6 and 6.7 show that major lower-
ing of the 6~80 values in carbonates occurred both at St.
Barthelemy and at Castillon and that this was also accom-
panied by some isotopic homogenization. However, the
data from Agly and Lapege are rather different. Here the
INTO values of the carbonates span a wide range from +13
to +22, including both values typical for sedimentary car-
bonates (as in Zone 1) and values characteristic of the
homogenized and ~8O-shifted marbles of Zone 2.
In order to investigate these systematics further, we
made detailed isotopic profiles across some of the individ-
ual carbonate units in the Zone 3 basal gneisses. One such
profile from the Agly Massif is shown in Figure 6.8. The
OCR for page 102
1kJ~
-1
+22
+2O
+18
+16
~ 180
+14
+8
+~
+24 ZONE 1 | ZONE 2 | ZONE 3
_~\
_~
1~!
\ ~
~ it,
~1
/~' ,
1 ~\A xx1
+2 ; w ~ G {,, ~
~0~ /~ Ad/ v
/
FIGURE 6.6 Summary of the /0 data for metacarbonate
lithologies from the Pyrenees (excluding the Anze data shown in
Figure 6.4~. The calculated whole-rock carbonate 6~80 values
(i.e., including the coexisting silicate minerals) are plotted as a
function of metamorphic grade, with the relative width of each
lithological unit along the abscissa being proportional to the total
areal extent of each Ethology in the Castillon (Cast.), St. Barthel-
emy (St. B.), Trois Seigneurs (T.S.), and Agly massifs. Within a
given lithological subdivision, the data points are plotted propor-
tional to the actual geographic distance of the sample locality
from the boundary of this subdivision in the field. Silicates were
not analyzed for two Castillon samples and one low-grade St.
Barthelemy sample (marked with plus signs); these 6~80 values
represent calcite rather than whole rock. In the Trois Seigneurs
field two of the data points with error bars represent average
values (and the 6~80 range) for the two profiles shown in Figures
6 and 7 of Wickham and Taylor (1987~. The field of whole-rock
6~80 values of pelites, granites, and gncisses from all regions
(Figure 6.7) is shown for companson. Note the preservation of
relatively high (sedimentary) 6~80 values in Zone 1, the ~80-
depleted and homogeneous isotopic compositions of both the
carbonates and the silicate lithologies in Zone 2, and the wide
range of 6~80 values in all rock types in Zone 3.
calcites in these carbonate layers have essentially retained
their original sedimentary 6~80 values, indicating a lack of
exchange with the adjacent biotite gneisses (which have
6~80 values of about +121. Steep isotopic gradients of as
much as 10 per mil over 50 cm are preserved at the mar-
gins of the carbonates. These systematics occur despite
the fact that the Agly units are mostly substantially thinner
than those at Trois Seigneurs and were metamorphosed at
higher temperatures (both of which factors might be ex
STEPHEN M. WICKHAM AND HUGH P. TAYLOR, JR.
pected to favor isotopic homogenization). Similar sys-
tematics are observed in the Lapege samples. Again, steep
isotopic gradients, internal isotopic heterogeneity, and lack
of homogenization with the adjacent lithologies character-
ize the carbonate unit sampled here (see Wickham and
Taylor, 1987; Figure 6.8~. This clearly implies a different
style of fluid-rock interaction in the Agly and Lapege
Zone 3 rocks, as compared with those in Zone 2 in the
same areas. In this respect it is important to note that there
is no obvious contrast in the present-day permeabilities of
Zone 2 and Zone 3 rocks, although major contrasts may
have existed between these lithologies at the time of active
Hercynian metamorphism.
At St. Barthelemy and Castillon, the oxygen isotope
systematics in the Zone 3 basal gneisses have much more
in common with the higher-level Zone 2 rocks. Metacar-
bonates from these regions have 6~80 values similar to
those for the Zone 2 rocks from Trois Seigneurs, and the
steep isotopic gradients that are a characteristic feature at
+22
8'~C
. ZONE I Zn~F ~I ZONF ~
· ,
At,
+8 _
+6 _
+429 I I ~ ~1 )
~~v + ~O
FIGURE 6.7 Summary of the whole-rock /0 data for pe-
lites, granites, and gneisses plotted as a function of metamorphic
grade (as described in the caption for Figure 6.6~. The data
points lying above the heavy black bar on the abscissa represent
values for mafic rocks within the basal gneisses. The field for
the whole-rock carbonate samples plotted in Figure 6.6 is also
repeated in this figure for reference. Note the strong lowering of
6'80 in all lithologies (including the carbonates) going from
Zone 1 to Zone 2. The homogeneous 6'80 values in Zone 2
reflect the infiltration of large volumes of aqueous fluid into all
lithologies at high temperatures. The 6'80 values are more
heterogeneous in Zone 3, indicating that the Zone 3 rocks did not
experience such a massive water influx.
OCR for page 103
HYDROTHERMAL SYSTEMS ASSOCIATED WITH REGIONAL METAMORPHISM AND CRUSTAL ANA TEXIS
AG LY
12
10
8
2
_ _
x Gneiss whole rocl' O Silicate residue
O Colcile ~ Calculoted carbonate shore rock
/~///X~//I OT ~ T E /////
o DO (70%) -
CARBONATE
~ - A~5%}
//////////.
~ ~(91 To)
CARBONATE O 0(
O(84~)
L~//~//~/////~/~/:
+16 tt8 +20 +22
81So
Agly and Lapege have not yet been identified. Some
isotopic heterogeneity does occur between the rare mafic
rocks in these areas and the more abundant biotite gneisses,
but it is not as marked as that observed between the car
bonates and gneisses at Agly.
In general, isotopic heterogeneity within the basal
gneisses is more extreme than is observed anywhere in
Zone 2, suggesting that the Zone 3 rocks throughout the
Pyrenees were not subjected to the same isotopic homog
enization process that occurred at higher structural levels.
This increase in isotopic heterogeneity with increasing
structural depth in the Hercynian crust strongly implies
that fluid movement during metamorphism became less
important at deeper structural levels.
Interpretation of Zone I and Zone 2 Oxygen Isotope
Systematics
There are essentially two plausible low-~8O reservoirs
available to account for the lowering of INTO in going from
Zone 1 to Zone 2. One is mantle-derived lower crustal
rocks, which would probably have had 6'8O values be
tween +6 and +10. The other is a large volume of low-'8O
aqueous fluid. Geological evidence in the Pyrenees favors
the latter reservoir because suitable low-'8O rocks are
notably absent from the wide range of structural levels
exposed. Basal gneisses in Zone 3 contain small amounts
of mafic rock with 6'8O values of +6 to +8, but are domi
103
FIGURE 6.8 Oxygen isotopic profile
through two thin carbonate layers within
granulite-facies quartzo-feldspathic gneiss
in the Agly Massif (see Figure 2 of Wick-
ham and Taylor, 1987, for locality). The
weight percent calcite in each carbonate
sample is shown. Despite the small di-
mensions of the carbonate layers (<3 m
thick), most of the calcites have essen-
tially retained their original sedimentary
INTO values, indicating a lack of /0
exchange with the adjacent gneisses. The
original INTO values of these gneisses are
not known, but they could have been nearly
identical to their present metamorphic
values (see text).
nantly composed of metasedimentary biotite gneisses with
6'8O values of +11 to +12. Late granodiorite plutons have
values typically in the range +8 to +11, but these were
intruded after Hercynian metamorphism and were not
available to take part in the earlier isotopic homogeniza-
tion processes. The synmetamorphic peraluminous grani-
toids were themselves mainly derived from politic metasedi-
mentary material (Wickham, 1987a) and have average 6'8O
values of +11 to +12. A suitable low-'8O rock reservoir is
therefore lacking at any exposed level of the Hercynian
crust. Furthermore, the isotopic data from the Zone 3
rocks at Lapege and Agly imply an increase in the degree
of isotopic heterogeneity with increasing structural depth.
This is opposite to what would be expected if the Paleo-
zoic metasediments were being isotonically homogenized
by some large-scale process involving interactions with
low-'8O materials from the mantle or the lower crust.
If the low-'8O reservoir were an aqueous fluid of some
type, then simple material-balance calculations could be
used to place constraints on the quantity of fluid involved.
Clearly, this depends on the original 6'8O value of the
fluid and the temperature of the isotopic exchange as well
as the magnitude of the isotopic shift observed in the
rocks. This is illustrated in Figure 6.9, where water-rock
ratio is plotted against d'8O, the initial 6'8O value of infil-
trating fluid (see Taylor, 1977, and Wickham and Taylor,
1985, for details), assuming a shift in 6'8O of 3.5 per mil
between Zone 1 and Zone 2 for the bulk terrane. Exchange
