Geodinamica Acta 15 (2002) 45–61
www.elsevier.com/locate/geoact
Geodynamic evolution of the South Variscan Iberian Suture as recorded
by mineral transformations
Jorge Figueiras a,*, António Mateus a, Mário A. Gonçalves a, João Waerenborgh b,
Paulo Fonseca c
a
Departamento de Geologia and CREMINER, Edifício C2, 5º piso, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisbon,
Portugal
b
Departamento de Química, Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal
c
Departamento de Geologia and LATTEX, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Edifício C2, 5º piso, 1749-016 Lisbon,
Portugal
Received 10 July 2001; accepted 15 December 2001
Abstract
New structural, petrographic, mineralogical and geochemical data from the Beja-Acebuches Ophiolite Complex (BAOC) are presented,
and reviewed together with data published elsewhere. The new data obtained shed light on questions such as: 1) the relative importance of
the obduction event; 2) its geological record in the deep levels of BAOC; 3) the nature and intensity of the Variscan metamorphism and
deformation during subsequent continental (arc) collision; 4) the age relationships between BAOC and the Beja Igneous Complex; and 5)
by means of numerical modelling, the thermal metamorphism of the Ossa-Morena autochthonous terranes induced by the ophiolite
obduction. The emerging picture is that of a fairly simple overall geological evolution for BAOC, seamlessly integrated within the evolution
of the southern branch of the Iberian Variscides. Obduction of BAOC is a relatively minor early event in the general NE–SW convergence
that gave rise to the orogen as seen regionally and is recorded by an anisotropic, high-temperature, metamorphic fabric at the gabbro levels
and by subtle features of the chemical composition of primary minerals at the underlying peridotite level; it caused chilling of the obducted
ophiolitic slab and no significant metamorphism on the autochtonous rocks of the Ossa-Morena Zone. BAOC underwent most of its
deformation and (amphibolite facies) metamorphism during a later collisional event, that took place as the most primitive rocks of the Beja
Igneous complex were being intruded, and whose waning stages are responsible for extensive serpentinisation of peridotites and for
important aquocarbonic fluid discharges along the semibrittle–brittle shear zones meanwhile developed. © 2002 Éditions scientifiques et
médicales Elsevier SAS. All rights reserved.
Keywords: Iberia; Mineral transformations; Ophiolite; Upper Paleozoic; Variscides
1. Introduction
The Hesperian Massif of Iberia is one of the most
continuous outcrops of the Variscides of Europe and its
study is important for the understanding of the southern
branch of this orogenic belt. Traditionally, the Iberian
Variscides have been divided in several longitudinal geotectonic zones, each characterised by a paleogeographic,
tectonometamorphic and magmatic history of its own.
Relatively recent studies have shown that the border be-
* Corresponding author.
E-mail address: jmvf@fc.ul.pt (J. Figueiras).
© 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.
PII: S 0 9 8 5 - 3 1 1 1 ( 0 1 ) 0 1 0 7 8 - 6
tween the two southernmost of these zones (the OssaMorena and the South Portuguese Zones) is lined up by a
long (circa 130 km) and narrow (maximum width 1.5 km)
belt of mafic and ultramafic rocks, which is currently
interpreted as a dismembered and scattered ophiolite complex placed along a suture formed during accretion of the
exotic terranes of the South Portuguese Zone onto the
autochthonous or parautochthonous terranes now incorporated in the inner zones of the orogenic belt [1,2]. This belt
of mafic–ultramafic rocks, known as the Beja-Acebuches
Ophiolite Complex (BAOC), is most of the times affected
and bounded by WNW–ESE shear zones and/or their late
subsidiary structures, whose development can be ascribed to
the late stages of continental collision. In the present work,
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J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
structural, petrographic and chemical data collected at or
near these tectonic accidents will be presented, and their
implications to the post-emplacement evolution of the
ophiolite sequence and the suture wherein it occurs will be
discussed.
2. Previous work and geological background
The current identification of the mafic and ultramafic
rock units included in BAOC as an ophiolite complex is not
straightforward and stems from geochemical considerations, coupled to lithostratigraphic integration of rocks
scattered along the whole length of the belt.
Geochemical data pointing to an ‘ophiolitoid’ nature of
the portuguese part of BAOC rocks were first published by
Andrade [3–5]. This somewhat controversial initial
geochemical evidence has later been supplemented by the
work of Munhá et al. [6,7], which improved our understanding of the oceanic geochemical affinities of BAOC and
showed that these affinities were shared by rocks outcropping both in Portugal and in Spain (the so-called Acebuches
amphibolites). Despite its geochemical affinities, controversy over the ophiolitic nature of BAOC went on, because
the lithostratigraphic column normally characterising ophiolites is nowhere to be found. Only after field work summarised by Fonseca and Ribeiro [8] and Quesada et al. [2],
did the ophiolitic nature of BAOC gain widespread acceptance, for it has been demonstrated that all rock units
normally belonging to ophiolite sequences can be found
within BAOC, and that the original spatial relationships of
those rock units have been destroyed by the development of
several very important longitudinal shear zones, which
brought to contact rocks originally far apart from each other
in the lithostratigraphic sequence and reduced to one or two
the number of rock units to be found at any specific location.
It is now clear that in the eastern (spanish) part of the
belt, upper ophiolite sequence rocks predominate, whereas
in Portugal gabbros and peridotites are the main rock types
present. Examples of sheeted-dyke complexes and of sea
floor sediments are rare all over the belt. Their scarcity
needs no special explanation for they can easily have been
suppressed by tectonic lamination and it is now known that
these lithologic types may be absent from some ophiolite
sequences. BAOC is not a continuous belt bordering the
Ossa-Morena Zone to the South, its outcrops are interrupted
for several tens of kilometres by the Ficalho Fault, a
seemingly very important ENE–WSW transverse fault still
awaiting complete structural characterisation (Fig. 1).
For most of its portuguese length, BAOC runs along the
southern border of a large, mostly gabbroic, intrusive
complex (the Beja Igneous Complex, BIC [5,9,10]). Both
BIC gabbros and BAOC display mineral fabrics with
similar directions. Although in BIC the fabrics are carried
by the early magmatic mineral suite and in BAOC by a late
metamorphic
assemblage
denoting
lower
amphibolite/greenschist facies conditions, the geometrical
congruence of both fabrics forstered recent [11,12] support
of an old [4] hypothesis stating that the Portuguese tract of
BAOC is just the southern border of the Beja Igneous
Complex deformed in the vicinity of the Ferreira-Ficalho
Thrust. The latter is the most important accident of the
Southwest Iberia Suture Zone, for it runs continuously
(except when interrupted by the Ficalho Fault) between the
Tagus and Guadalquivir Cainozoic basins, separating the
low grade metasedimentary rocks traditionally ascribed to
the South Portuguese Zone to the South from BAOC and the
Ossa-Morena Zone to the North.
The northern border of BAOC is most of the times of
tectonic origin. It is underlined in Spain by a high grade
tectonic mélange ([2] and references therein), but in Portugal the exact nature of the contact is often difficult to assess,
due to poor exposure and lack of lithologic contrast between
BAOC and BIC. Although in a few instances, the contact of
BAOC and BIC can be shown to be intrusive [13], most of
the times it is inferred to be tectonic, on the basis of both its
straight cartographic expression and steep gravimetric and
magnetic gradients showing straight contour lines, observed
at the contact location (Instituto Geológico e Mineiro,
unpublished maps and reports). BAOC rocks experienced
metamorphism, but the apparent metamorphic grade is
variable from location to location and may show quite large
differences on both sides of the shear zones internally
affecting the ophiolite complex. Evidence for high grade
metamorphic recrystallisation [2] is sometimes present in
the mafic units; low-to-median grade metamorphism is
ubiquitous, retrograding the previous high grade mineral
assemblages when these are present. Some of the shear
zones running along BAOC are associated to extensive
carbonate and silica metasomatism.
Quesada et al. [2] have summarised all geological data
then available on BAOC. According to them, the geological
history of BAOC began as new transitional oceanic crust
formed in a marginal basin associated to north-directed
subduction of main oceanic lithosphere under the OssaMorena Zone. Soon after magmatic crystallisation, the
newly formed oceanic crust was obducted over the OssaMorena Zone (locally represented by a sedimentary sequence capped by a carbonate platform), as testified by D1
deformation phase structures [8]. After obduction, the deformation style changed to ESE–WNW thrusting and shear
development (D2 deformation phase), reflecting the onset of
continental (arc? ) collision. The final stages of evolution
are characterised by SW high angle thrusting reworking
former D2 structures. According to the above-mentioned
authors, the first metamorphic event recorded in BAOC
rocks (coeval with D1 deformation phase) reveals recrystallisation under what they termed low-pressure granulite
facies conditions, subsequent metamorphism being a retrogradation of this early high-temperature episode. For a
recent review on the subject, see [14].
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
47
Fig. 1. A. Simplified tectonostratigraphic terrane map of the Iberian Massif (adapted from [28]): 1 – Proterozoic to Upper Paleozoic Iberian Autochthonous
Terranes (a. Central Iberian Autochthon; b. Ossa-Morena Autochthon); 2 – Paleozoic Oceanic Terranes; 3 – NW Iberia Continental Allochthonous Terranes;
4 – South Portuguese Terrane. B – Simplified geological map of the Évora-Beja domain (adapted from [2]): 1 – Late Proterozoic to (Lower) Paleozoic
imbricated units of the Ossa-Morena Zone; 2 – Mafic and ultramafic rocks of the Beja-Acebuches Ophiolite Complex; 3 – Paleozoic autochthonous rocks
of Ossa-Morena Zone; 4 – Mafic and intermediate plutonic rocks of the Beja Igneous Complex; 5 – Acid plutonic and subvolcanic rocks of the Beja Igneous
Complex; 6 – Pulo do Lobo metasedimentary group; 7 – Cenozoic sedimentary deposits.
3. Data summary
3.1. Petrography
Overall, peridotites do not occupy large areas within
BAOC and occur mainly near major shear zones, amid
regions where gabbroic rocks prevail. Because of their close
association to the main tectonic accidents, these rocks are
strongly affected by metasomatic and mineralogical transformations induced by late fluid circulation along the shear
zones. The peridotites of BAOC underwent most of the
times extensive polyphasic serpentinisation, but the usual
presence of relics of Cr-spinel, Mg-rich olivine, enstatite
and, sometimes, diopside, strongly suggests harzburgite as
the original rock. However, in some places, only spinel
relics are observed and this is taken to point to a dunitederived serpentinite. Rare wehrlites/troctolites are also
known from the same general tectonic setting of BAOC
ultramafic rocks; they show incomplete serpentinisation
and, besides abundant An70 plagioclase, significant amounts
48
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
of disseminated sulphides, mainly pentlandite and pyrrhotite; their original relationships with the rest of the peridotites are presently unclear.
Gabbroic rocks outcropping in Portugal are mostly gabbros and gabbronorites, composed of An51-An60 plagioclase, relics of diopside, brown Na- and Ti-bearing hornblende, ilmenite, titanium-rich magnetite and some
sulphides (mostly pyrrhotite, chalcopyrite and pyrite); badly
preserved relics of olivine are sometimes observed. These
rocks are metamorphosed in a transitional greenschistamphibolite facies, whose typical mineral assemblage,
albite + oligoclase + actinolite + pale-brown
to
green
hornblende + epidote + chlorite + sphene [2], is particularly
well developed in the uppermost section of the ophiolite
sequence. In deeper-lying rocks, this metamorphic episode
is represented by the transformation of primary pyroxene
and amphibole to green hornblende and actinolite, by
quartz ± chlorite deposition and by the development of
reaction rims of untwinned plagioclase around earlier deformed crystals of this mineral; these mineral transformations may obliterate evidence for an earlier syntectonic
recrystalisation event under very high temperatures (800900ºC) and pressures under 5 kbar, as attested by the
presence of Ca-plagioclase, olivine, ortho- and clinopyroxene and by the absence of coronitic garnet-spinel around
plagioclase [2]. In thin section, these rocks show several
superimposed deformation and metasomatic events. Plagioclase grains are commonly interlocking and show bent or
kinked twin lamellae, deformation bands, wavy extinction
and segregation bands; evidence for plagioclase subgrain
development or annealing was not detected. The observed
twinning is believed to be of mechanical origin, since twin
bands commonly wedge out within the grains and are
sometimes restricted to the margins of the grains in places
where stress concentration might be expected. In rocks
affected by the regional shear zones, plagioclase crystals
show cataclasis and later saussuritisation, whereas quartz
display strong plastic deformation followed by dynamic
recrystallisation.
In BAOC, metasomatism and hydrothermal alteration
decrease as a function of distance to the main shear zones
affecting the complex. The shear zones themselves are
underlined by relatively thin belts (width 10-20 m) of very
strong carbonatisation, usually accompanied by significant
silicification, which can be seen to wane outwards towards
rocks progressively less affected by carbonate or silica
deposition. The very serpentinisation, although ubiquitous,
seems to increase towards the accidents, whenever adequate
data and/or exposure exist.
In general, the mineral transformations that occurred
along the major shear zones can be described as a(an
almost) complete breakdown of the primary minerals and
their substitution by a secondary assemblage comprising
carbonates (mostly dolomite + ankerite, sometimes also siderite, magnesite and calcite), microcrystalline quartz, chlorite, rare pyrite grains and some ill-crystallised iron and
titanium oxides. The more resistant primary minerals
(chrom spinel and ilmenite + plagioclase in the case of
peridotites and gabbros, respectively) are commonly preserved, and the lithologic nature of the protolith causes
differences in the secondary mineral assemblages, which
include, for peridotites, serpentine ± magnesite, with textures showing that carbonatisation postdates serpentinisation, and leucoxene ± sericite ± siderite, for gabbroic rocks.
Moreover, when (relatively late) polyphasic veins are
present within the strongly carbonatised domains, it can be
seen that their earliest infillings depend on the host lithology, being predominantly sideritic when hosted by mafic
rocks and silicic or dolomitic when hosted by peridotites.
The final stages of carbonate deposition are invariably
represented by veins and vugs filled by coarse, sparry
calcite.
The sequence of mineral transformations having occurred along the major shear zones can be studied at some
distance from them, where the original (more or less
serpentinised) rocks are still partially preserved. In the
peridotites, incipient carbonatisation enhanced the formation of ferritchromite ± magnetite reaction rims around
chromspinel grains, which had begun during prior serpentinisation. In the gabbroic rocks, this initial stage of alteration leads to decomposition of the mafic minerals and to
the precipitation of chlorite. The onset of strong carbonatisation is marked by the inception of felsic silicate decomposition, all silicate minerals (including serpentine) being
replaced by fine-grained isotropic aggregates of dolomite
and ankerite (± siderite, in the case of gabbroic rocks), by
sulphide oxidation and by deposition of significant amounts
of silica as micrometric (often corroded) quartz, probably a
by-product of serpentine decomposition; millimetric vugs
filled with quartz testify a late silicification.
3.2. Whole rock geochemistry
Average chemical compositions of non-metasomatised
BAOC lithologic types are quoted in Table I, together with
the variation range observed for each chemical element.
Despite of the presence of rocks that are petrographically
classified as wehrlites/troctolites and as primitive gabbros,
rocks of intermediate composition between the two groups
just mentioned are very rare (Fig. 2).
Peridotites are characterised by the presence of a clear
differentiation trend, which is shown by variable (and low)
alumina concentrations positively correlated with such
chemical components as Na2O and CaO and negatively
correlated with MgO and Ni. In this group, Ti concentrations are low and independent of the alumina contents,
whereas V has quite variable values, poorly correlated with
Al2O3. The differentiation trend of the ultramafic rocks is
also well expressed in their minor elements concentrations,
with Cr and Co co-varying with Ni, and V and Zr with Ti.
As expected, iron and Ti are totally uncorrelated, as are also
Cu and Ni.
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
49
Table I
Representative analyses of non-metasomatised BAOC rocks.
Peridotites
AZM1
Wt%
SiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
TiO2
P2O5
LOI
Total
%
S
C
37.71
8.68
9.21
0.13
28.90
4.04
0.41
0.01
0.08
0.01
10.81
99.99
0.040
0.325
Ppm
Ba
Sr
Y
Zr
V
Cu
Zn
Ni
Co
Cr
Sc
W
La
Ce
Nd
Sm
Eu
Tb
Yb
Lu
10
89
1
5
15
33
31
1286
108.0
2210.0
6.9
54
0.3
1
1
0.14
0.14
0.1
0.17
0.03
Gabbroic rocks
FA-13
35.60
0.97
17.39
0.19
34.43
0.77
0.04
0.01
0.08
0.01
10.71
100.20
0.155
0.094
3
9
2
6
18
62
78
1217
133.0
1180.0
9.4
8
0.3
1
1
0.19
0.08
0.1
0.23
0.03
n = 8(*)
(35.51-39.93)
(0.97-8.73)
(9.21-17.39)
(0.13-0.19)
(27.38-36.71)
(0.12-5.12)
(0.01-0.81)
(0.01-0.81)
(0.07-0.31)
(0.01-0.07)
(7.94-13.62)
(0.02-0.16)
(0.06-0.33)
(3-78)
(2-136)
(1-6)
(5-23)
(5-58)
(33-97)
(31-78)
(759-2181)
(96.7-148.0)
(1180-3620)
(6.4-13.5)
(6-54)
(0.2-2.3)
(1-6)
(1-3)
(0.14-0.88)
(0.08-0.70)
(0.2-0.5)
(0.17-0.70)
(0.03-0.11)
STP15
MAC2
n = 7(*)
46.49
18.82
7.36
0.12
4.36
12.87
2.84
0.12
2.29
0.26
3.16
98.69
49.15
20.07
7.46
0.14
5.67
9.58
3.72
0.47
1.90
0.07
1.92
100.15
(44.08-56.98)
(10.24-20.07)
(7.10-12.08)
(0.11-0.17)
(4.36-11.65)
(7.00-12.87)
(2.40-4.98)
(0.12-0.62)
(0.83-3.54)
(0.02-0.26)
(1.92-8.34)
0.068
0.578
69
364
7
9
422
38
37
24
44.0
74.9
37.3
109
3.8
9
5
1.28
0.79
0.2
0.74
0.11
0.010
0.239
165
437
6
10
314
17
36
21
43.7
45.0
36.3
121
2.8
7
4
0.96
0.96
0.2
0.72
0.11
(0.01-0.07)
(0.15-1.51)
(64-258)
(100-437)
(6-38)
(9-291)
(164-595)
(11-38)
(36-54)
(13-143)
(32.9-70.5)
(40.9-235.0)
(28.6-67.2)
(41-227)
(2.8-38.5)
(7-58)
(4-21)
(0.96-5.44)
(0.59-1.58)
(0.2-1.8)
(0.74-2.48)
(0.11-0.52)
* total number of analysed samples and respective range of analytical data for each oxide or element
As is the case for most ophiolitic sequences, BAOC is
quite poor in rocks more silicic than gabbros or basalts.
According to this, its mafic rocks show no discernible
differentiation trend when its major element compositions
are examined. This is however not true for minor element
compositions, where some trends involving Ni, Cr, Ti, V
and, to some extent, also Zr, span both gabbroic rocks and
peridotites without any break, which is remarkable given the
presence of a very distinct gap in major element concentrations between both types of rocks (Fig. 3).
Many of the covariant trends of BAOC rocks are readily
explained by their mineralogy. For instance, the outstanding
Ti-V correlation in gabbroic rocks is mainly caused by
ilmenite accumulation and the co-variance of Ni and Cr in
the peridotites is due to the simultaneous presence of
Cr-spinel and sulphides (mainly pentlandite + pyrrhotite) or
Ni-bearing olivine.
In BAOC, amphibolites and evolved gabbros display
parallel REE normalised patterns, showing small negative
Eu anomalies, which, coupled to evident Sr depletions,
indicate crystallisation from a liquid having previously
precipitated plagioclase and/or clinopyroxene under reducing conditions. Indirect support to this interpretation comes
from the observation that primitive gabbros within BAOC
are enriched in both Ba and Sr and have large positive Eu
anomalies. REE patterns of peridotites display a more or
less distinct positive Eu anomaly, but their interpretation is
not straightforward, since they may result from the presence
of earlier plagioclase (observed in wehrlites/troctolites but
impossible to prove for wholly serpentinised rocks) or from
the serpentinisation process itself, e.g. [15,16].
That BAOC is not a typical mid ocean ridge igneous
complex is indicated by the fact that many of its basic rocks
have Zr concentrations around 100 ppm, which, according
50
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
Fig. 2. Major elements whole-rock composition of BAOC rocks. Open squares, ultramafic rocks; cross signs, idem retrieved from [5]; open triangles,
metagabbros; plus signs, metagabbros retrieved from [5]; black dots, amphibolites retrieved from [5] and [2]; open hourglasses metabasalts retrieved from
[2].
to Pearce and Cann [17], suggests an environment more
similar to a marginal oceanic arc than to a typical spreading
ridge. The same is shown by the plots Ti – V, Zr/Y – Zr and
Ni – Ti/Cr (Fig. 3), where the points representing BAOC
rocks consistently fall within or at the margins of the field
occuppied by island arc basalts [18,19,20].
The metasomatic processes that occurred along the main
shear zones affecting BAOC (mainly carbonatisation and
silicification) have resulted in a general leaching of most of
the major elements of the original rocks (Ca being the
outstanding exception) but have preserved to some extent
the original chemical signature of the rocks as displayed by
their minor element compositions. Examples of this are the
high and positively correlated Ti and V concentrations
displayed by altered rocks derived from gabbros and the
high values of Cr, Ni, Co, and often also Cu, found when the
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
51
Fig. 3. Minor elements whole-rock composition of BAOC rocks. Symbols as in Fig. 2.
metasomatised rock derives from an ultramafic protolith. A
detailed discussion of the metasomatic processes in BAOC
shear zones can be found in Mateus et al. [21].
3.3. Mineral chemistry
Tables II and III report representative analyses of BAOC
primary and secondary minerals, respectively. In nonmetasomatised rocks, minerals other than Cr-bearing spinel
have slightly variable chemical compositions but no significant zonation is observed in any of the minerals present.
Olivine compositions are quite homogeneous throughout
but two different types can be distinguished on the basis of
their Ni contents: olivines coexisting with abundant orthopyroxene and/or with high-temperature Ni-bearing sulphide
minerals are depleted in Ni when compared with olivines
occurring in other contexts.
Spinels (other than magnetite) in BAOC are restricted to
peridotites and wehrlites/troctolites. In the diagram
Cr/(Cr + Al) – Fe/(Fe + Mg) (Fig. 4) they plot distinctly to
the right of the main spinel compositional trend seen in the
data compiled by Roeder [22] and also to the right of the
52
Table II
Representative chemical analyses for different primary mineral species (expressed as oxide wt%) and cation proportions.
Olivines
Pyroxenes
AZM1 PLM5 N = 18
PLM2 STP15
n = 85
39.51 (38.17-40.56) 54.89
0.00 (0.00-0.04)
0.34
0.00 (0.00-0.02)
1.18
51.99
0.43
1.99
0.25
(40.10-46.99) 30.34
(0.00-0.04)
1.53
(0.10-0.30)
0.20
(11.55-19.90) 9.41
(0.14-0.33)
0.04
0.03
13.93
22.30
0.16
8.82
0.00
44.69
0.02
0.19
14.36
0.14
98.92
0.05
0.34
0.00
0.00
98.24 100.00
1.00
0.00
0.00
0.00
0.00
1.96
0.01
0.05
1.93
0.01
0.09
0.01
0.00
1.69
0.00
0.00
0.30
0.00
0.00
0.00
0.00
3.00
1.62
0.06
0.01
0.28
0.00
0.77
0.89
0.01
0.27
0.00
0.00
0.00
4.00
0.02
0.00
4.00
Plagioclases
Spinels
N = 15
PLM4 MEL11 MAC2 N = 11
STP13 MAC4 N = 18
n = 45
n = 34
n = 115
AZM2 PLM2 FA1
(51.20-54.84) 41.88 42.24 47.67 (41.67-49.29) 53.00 54.29 (47.33-55.24)
(0.20-0.60)
3.46 1.70 1.37 (0.55-3.46)
0.37
(1.18-2.38)
11.61 8.14 7.15 (6.11-13.92) 28.93 29.13 (27.32-32.08) 30.25
0.20
(0.00-0.32)
1.43 0.07 0.02 (0.00-1.87)
31.39
(13.51-31.00) 15.82 11.34 14.35 (11.34-16.56)
12.32
(0.39-22.92) 10.72 10.35 11.26 (10.13-14.55) 12.46 11.15 (10.20-15.72)
(0.16-0.25)
0.10 0.18 0.22 (0.08-0.32)
0.30
(4.88-10.18)
6.35 15.56 13.34 (4.96-15.56)
0.31 0.29 (0.00-0.65)
23.16
(0.00-0.04)
0.16
0.16
(0.01-0.39)
2.98 1.30 1.26 (1.04-3.10)
4.39 4.80 (1.04-3.10)
(0.00-0.01)
0.40 0.30 0.20 (0.18-0.48)
0.17 0.12 (0.01-0.32)
94.75 91.19 96.84
99.25 99.78
98.31
6.23
0.39
2.04
0.00
0.17
6.74
0.20
1.53
0.00
0.01
7.02
0.15
1.24
0.00
0.00
3.51 2.70 3.15
1.71 1.77 1.78
0.01 0.02 0.03
0.79 2.08 1.64
0.01 0.00 0.00
0.00 0.00 0.00
0.86 0.40 0.36
0.08 0.06 0.04
15.78 15.52 15.41
9.68
9.81
6.23
6.20
0.05
0.04
2.44
2.16
1.55 1.68
0.04 0.03
19.98 19.92
Ilmenites
N = 19
MAC4 STP15 N = 10
n = 121
n = 77
0.01 0.52 (0.00-2.28)
48.44 43.18 (39.28-49.28)
33.67 27.35 (20.75-44.84)
0.05 0.32 (0.04-0.48)
1.83 2.44 (1.09-2.98)
29.67 31.28 (10.17-35.04)
9.74 7.49 (6.37-13.47)
0.10 0.05 (0.01-1.13)
0.31 0.38 (0.23-0.59)
1.94 0.65 (0.00-2.67)
24.97 30.12 (19.19-38.29) 43.25 52.35 (41.65-52.35)
0.10 0.13 (0.00-0.51)
0.46 0.35 (0.00-0.67)
98.98 97.94
0.07
8.61
0.04
6.00
1.27
4.44
0.00
9.59
0.01
5.67
0.71
3.51
0.10
8.15
0.06
6.25
1.42
2.82
0.06
3.41
0.03
0.03
0.06
4.34
0.02
0.08
95.56 98.67
1.91
1.64
0.08
0.10
0.08
0.01
0.60
0.00
0.08
4.94
0.03
0.06
0.09
1.88
0.03
1.61
23.96 23.99 23.41
4.05
3.98
§ Calculations on the basis of: 3 cations for olivine; 4 cations for pyroxenes and ilmenites; (15-Na,K) or (13-K,Na,Ca) cations for amphiboles and 32 oxygens for plagioclases and spinels.
N = number of examined samples; n = number of analyses.
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
SiO2 39.85
TiO2 0.02
Al2O3 0.00
V2O3
Cr2O3
MgO 46.02
CaO
0.00
MnO 0.16
FeO 13.08
NiO
0.33
ZnO
Na2O
K2O
Total 99.46
§
Si
1.00
Ti
0.00
Al
0.00
V
0.00
Cr
0.00
Fe 3+
Mg
1.72
Ca
0.00
Mn
0.00
Fe 2+ 0.27
Ni
0.01
Zn
0.00
Na
0.00
K
0.00
Total 3.00
cations
Amphiboles
Table III
Representative chemical analyses for different secondary mineral species (expressed as oxide wt%) and cation proportions.
Amphiboles
MAC1
Chlorites
AZM13
N = 10
PLM2
Plagioclases
MEL17A N = 14
n = 33
AZM9
n = 42
N = 18
AZM10
Ti-0xides
STP10A1 N = 5
n = 44
54.18
0.34
2.07
55.15
0.18
1.19
(42.40-56.92) 29.94
(0.00-0.64)
0.02
(0.28-23.94) 17.65
26.49
0.02
20.02
(25.45-33.87) 64.37
(0.00-0.15)
(8.92-24.70) 21.57
0.00
17.71
11.52
0.20
10.48
0.02
21.85
1.08
0.43
16.61
(0.00-0.06)
(0.24-24.60)
(0.75-27.35)
(0.00-0.59)
(0.25-18.27)
0.00
25.94
0.08
0.09
9.44
0.00
11.87
0.06
0.04
25.89
(0.00-0.34)
(0.01-30.60)
(0.00-1.08)
(0.01-17.09)
(7.99-28.33)
0.31
0.06
96.87
0.12
0.10
96.73
(0.01-0.86)
(0.00-0.17)
0.44
0.71
84.31
0.00
0.01
84.39
(0.00-1.81)
(0.00-1.79)
7.76
0.04
0.35
7.89
0.02
0.20
3.04
0.00
2.11
2.69
0.00
2.39
0.00
0.00
0.00
0.00
3.78
1.77
0.02
1.26
4.66
0.17
0.05
1.99
3.92
0.01
0.01
0.80
1.80
0.01
0.00
2.20
0.09
0.01
15.08
0.03
0.02
15.02
0.09
0.09
10.07
0.00
0.00
9.09
55.60
(53.00-64.85)
28.08
(21.56-29.69)
3.20
8.71
(2.25-9.87)
0.03
0.31
(0.00-1.16)
9.69
0.12
98.97
5.08
0.40
98.18
(3.26-9.69)
(0.06-2.53)
11.46
10.12
4.52
6.03
0.00
0.05
0.61
1.70
3.34
0.03
19.97
1.79
0.09
19.78
AZM12
MAC4
STP14
n = 28
N = 12
n = 52
0.46
1.30
0.18
32.51
0.19
2.00
0.67
0.37
31.86
0.01
(0.29-3.97)
69.67
(0.67-13.28)
(0.11-0.78)
4.46
(15.88-36.03)
(0.01-6.38)
0.22
24.70
79.34
(24.70-93.81)
1.20
4.18
(0.84-5.30)
0.01
0.02
(0.00-1.38)
0.34
39.48
0.23
13.03
0.35
56.49
0.38
0.52
(0.08-0.70)
0.34
(25.49-64.66) 10.69
(0.11-1.36)
(0.24-13.03)
0.00
64.47
0.06
5.20
(0.00-1.26)
(0.39-66.86)
87.71
92.66
85.37
90.38
88.79
0.12
0.51
0.05
8.64
6.60
0.10
0.47
0.25
0.09
7.85
7.25
0.01
0.10
4.50
0.06
3.24
0.00
0.00
23.91
0.09
7.47
0.10
0.12
0.00
0.00
23.70
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
SiO2
TiO2
Al2O3
V2O3
Cr2O3
MgO
CaO
MnO
FeO
NiO
ZnO
Na2O
K2O
Total
§
Si
Ti
Al
V
Cr
Fe 3+
Mg
Ca
Mn
Fe
Ni
Zn
Na
K
Total
cations
MAC7
Spinels
§ Calculations on the basis of: (15-Na,K) or (13-K,Na,Ca) cations for amphiboles; 32 oxygens for plagioclases and spinels and 14 oxygens for chlorites.
N = number of examined samples; n = number of analyses.
53
54
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
Fig. 4. Cr/(Cr + Al) – Fe/(Fe + Mg) plot of BAOC Cr-spinels. Points plotting above Cr/(Cr + Al) = 0.50 and above Fe/(Fe + Mg) = 0.70 represent altered
spinels included in strongly metasomatised samples.
theoretical high-temperature isopleth corresponding to the
coexisting olivine compositions [23]. Spinels included in
wehrlites/troctolites and in clinopyroxene-rich rocks, show
the largest deviation to the right and plot in an elongated
region instead of in a more or less circular and restricted
region as is the case for spinels included in peridotites.
Ilmenite occurs only in gabbroic rocks and is not
stoichiometric, having excess iron, low titanium and significant amounts of vanadium. In the structure of ideal ilmenite,
FeTiO3, Fe2+ and Ti4+ occupy two distinct sets of 6c equipositions corresponding to alternate octahedral layers in the
hexagonal close-packing sequence of the anions along
[001]. Single crystal X-ray diffraction shows that one of
these sets of octahedral layers is fully occupied by iron
while the other set of layers, the Ti layers of the ideal
ilmenite structure, is partially occupied by iron. Confirming
an Fe content larger than in ideal FeTiO3, the c0 lattice
parameter is also greater than the ideal value [24], thus
indicating solid solution of hematite (8% hematite, accord-
ing to data presented in Lindsley [25]). Mössbauer spectra
also confirm the presence of Fe3+ in the Fe-rich ilmenite. A
typical spectrum of an ilmenite concentrate, taken at 61 K,
is represented in Fig. 5. The spectrum shows the presence of
the doublets due to Fe3+ and Fe2+ in the ilmenite structure,
as well as Al-Ti substituted hematite and Fe2+ in the
structure of silicates [24]. Hematite is present as bulk
material. No hematite exsolution was observed microscopically and there is no X-ray diffraction evidence for superstructures due to periodic intercalation of consecutive
hematite-like layers (layers fully occupied by Fe) within the
ilmenite crystal structure.
In deformed gabbros, plagioclase usually displays conspicuous zonation with cores of An60-An70 composition
(that is, the normal composition of BAOC plagioclases)
surrounded by anorthite-poor rims of the same (oligoclase)
composition as non-twinned and undeformed plagioclase
aggregates filling late irregular microfractures and many
segregation bands within the primary plagioclase grains.
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
Fig. 5. Mössbauer spectrum of a BAOC ilmenite concentrate, taken at
61 K. The thick gray line component is due to Fe2+ in silicates and the
magnetic sextet (thin gray line) is due to Al,Ti-substituted hematite. The
two doublets plotted with black lines correspond to Fe3+ (thick line) and to
Fe2+ (thin line) in the ilmenite structure.
In the gabbroic rocks of BAOC, primary amphiboles are
corroded brown hornblendes characterised by significant
concentrations of sodium. Later amphiboles, formed during
Variscan metamorphism, are typically magnesium-rich
hornblendes, sometimes surrounding relic primary pyroxene grains, grading to actinolite-tremolite compositions as
metamorphic temperatures decreased. Chlorite composition
also differs according to the context of precipitation: chlorite coeval with the late serpentinisation pulses of
wehrlites/troctolites is tri-octahedral and iron-poor, whereas
chlorite deposited during carbonatisation along the regional
shear zones is mainly di-octahedral (maximum octahedral
occupancy 2.5) and has Fe/(Fe+ Mg) ratios up to 0.6.
Metasomatism (serpentinisation and carbonatisation)
does not lead to complete decomposition of the primary
Cr-spinel, but the composition of this mineral is strongly
altered during the process. The alteration may be thought of
as an aluminium and magnesium leaching (that in some
locations proceeds to completion), accompanied by considerable oxidation of the iron left behind and sometimes by
significant zinc incorporation. In some wholly carbonatised
rocks, petrographic observations and electron microprobe
data show the presence of completely and homogeneously
altered grains of spinel, with total leaching of Al and Mg
and a stoichiometrically estimated 60% of the iron atoms in
the Fe3+ state. Mössbauer spectra measured between room
temperature and 6 K show that there are several different
spinels present which can be distinguished by the amount of
Fe3+ that each one contains and by their different magnetic
ordering temperatures [26]. A typical spectrum, taken at
172 K, is represented in Fig. 6, showing several contributions. The sextet with the largest magnetic splitting is due to
Al,Ti-substituted hematite. The remaining contributions are
due to Fe in chromite grains. Both sextets with lower
magnetic splittings than hematite are attributed to magneti-
55
Fig. 6. Mössbauer spectrum of a BAOC Cr-spinel concentrate, taken at
172 K. The thick grey line component is due to Al,Ti-substituted hematite.
Both sextets with lower magnetic splittings than hematite are attributed to
magnetically ordered chromite. The doublet represented as a black thick
line, and the distribution of doublets represented as black thin lines, are
due, respectively, to octahedral Fe3+ and tetrahedral Fe2+ in paramagnetic
chromite.
cally ordered chromite. All doublets are due to Fe in
paramagnetic chromite, the one represented as a black thick
line to octahedral Fe3+ and the rest to tetrahedral Fe2+. The
spectra indicate that the dominant spinel types are those
which are paramagnetic down to 6 K and have a lower
amount of Fe3+, i.e., a significant part of the chromite grains
is still well preserved even in severally carbonatised rocks.
Mössbauer data, therefore, seem to indicate that Cr-spinel
partially withstood severe carbonatisation of its host rocks
and that several stages of alteration/oxidation may be
simultaneously present in the most altered samples.
Evidence for ilmenite alteration is widespread. In deformed but unmetasomatised rocks, ilmenite is partly preserved, but some of its grains have been replaced by a
crypto-crystalline aggregate of unidentified minerals, whose
global chemical composition as determined by electronmicroprobe analysis is Fe-poor and includes considerable
amounts of both Ca and Si, in agreement with the observation that plagioclase is being recrystallised with a more
sodian composition and quartz (as very fine matricial
aggregates) is being precipitated. In metasomatised rocks,
no ilmenite is preserved and most of the former ilmenite
grains are now pseudomorphic aggregates of ill-crystallised
titanium minerals, the remainder having been replaced by
Ti-bearing hematite. Mössbauer spectroscopy and X-ray
diffraction indicate that the mechanism leading to ilmenite
decomposition is iron oxidation. No evidence for hematite
intergrowths in the ilmenite structure was found. However
ilmenite seems to be present in different stages of alteration.
The average oxidation degree, 19% of the total Fe as Fe3+,
is being achieved by accomodating the excess Fe in the Ti
layers of the pure primary ilmenite. When the oxidation
degree of Fe becomes too large, the ilmenite crystal struc
56
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
ture seems to disrupt as suggested by the large number of
polycrystalline and poorly crystallized grains found. Fe is
segregated and forms Ti containing hematite which is
always present as a decomposition product, even when pure
separates of the pseudomorphs after ilmenite are taken for
analysis. Finally, it should be stressed that both Mössbauer
spectroscopy and X-ray diffraction reveal that «fresh»
ilmenite included in undeformed and unmetasomatised
rocks is really altered, all samples examined showing
several grains of the mineral replaced by a mixture of
hematite and titanium minerals. This is taken to represent
the effects of Tertiary weathering and must be taken into
account if mineral exploration for ilmenite is to be undertaken in BAOC.
3.4. Shearing within BAOC
The structure of BAOC is fairly complicated. The massive character of its rock units does not allow fold development, but the whole of BAOC is cut by several types of
non-coeval tectonic accidents. Most visible in the geological
maps are late NNE-SSW to NE-SW left-handed faults,
usually ascribed to the late phases of the Variscan deformation, that cut all rock units and tectonic features of BAOC,
and major E-W to WNW-ESE faults, of very complex
evolution, that separate BAOC from the adjoining geological domains and are responsible for the main features of its
present day architecture. Also important are brittle ENEWSW shear zones, that may form complex structural arrays
with the E-W to ESE-WNW accidents and some N-S to
NW-SE right-handed faults, commonly interpreted as the
conjugate system of the late Variscan tectonic accidents
mentioned above.
The detailed study of the mineral infillings and of the
hydrothermal alteration associated to the E-W to WNWESE shear zones [21] shows that they were initially formed
as semi-ductile shear zones during the waning stages of the
Variscan metamorphism and continued to develop as brittle
fault zones as deformation and metasomatism related to the
Variscan continental collision proceeded to completion.
Each shear zone is composed of several parallel and
anastomosed left-handed accidents with a significant souththrusting component, whose main traces have the same
general strike as the cartographic expression of the whole
shear zone and cut earlier, ductile-semiductile, pure lefthanded strike-slip, N140-N145 shear zones, which are at
present almost obliterated and show no or only incipient
carbonatisation coeval with their nucleation and propagation. Several minor families of movement planes can be
seen inside each shear zone, the significance of which is
difficult to ascertain, although most orientations, styles of
deformation and mineral infillings allow their interpretation
as conjugates of the main systems or as secondary fractures
formed at the tips of the propagating main shear zones.
The Ferreira-Ficalho thrust is also a left-handed, souththrusting WNW-ESE tectonic accident, but it is a special
case among the E-W to WNW-ESE shear zones because its
lateral continuity is exceptional, and its associated carbonatisation is exclusively calcitic. So, although the FerreiraFicalho Thrust is at present a major accident separating two
very different geological domains, comparison with the
remaining shear zones of similar orientation within BAOC
seems to indicate that it is a late accident with a comparatively simple evolution.
Detailed structural data on the whole of BAOC rocks has
been summarised by Quesada et al. [2] and shall not be
repeated here. Overall, tectonic deformation of BAOC is
strongly concentrated in the shear zones, but internal
deformation is clearly present in rocks away from the
tectonic accidents. In fact, in several portuguese locations
[13], a penetrative north-verging tectonic fabric is preserved
in the lower gabbroic section and is carried by the high
temperature metamorphic paragenesis already mentioned.
Also, many gabbroic rocks display a well marked stretching
lineation formed by the alignment of green
hornblende/actinolite crystals, precipitated during paroxysmal Variscan greenschist-amphibolite facies metamorphism.
This last lineation is the one geometrically compatible with
the magmatic fabrics observed in primitive gabbros of BIC.
4. Geodynamic evolution of BAOC an adjoining areas
As shown by their chemical composition and by what
remains of their original mineral paragenesis, BAOC rocks
were formed in an oceanic setting, probably in a back-arc
basin originating as normal oceanic lithosphere was being
subducted under the Ossa-Morena Zone.
Although the available evidence is very scant and somewhat controversial, there seems to have been progressive
magma mixing in some places during the original crystallisation of BAOC ultramafics. This is bore out by the
observation that BAOC ultramafics can be divided in two
subgroups: the first one includes mainly harzburgites and
dunites and contains Ni-bearing olivine and Cr-spinel of
homogeneous compositions; the second one, consisting of
clinopyroxene-rich peridotites and of troctolites/wehrlites,
is characterised by the presence of Ni-poor olivine, of
high-temperature sulphide minerals and of Cr-spinel of
variable chemical composition. The admixing magma causing the chemical variability of Cr-spinel [27] would also
have carried (due to crustal contamination in an arc environment?) the sulphur needed to precipitate the sulphide
minerals which sequestered the nickel that otherwise would
have been incorporated in the olivine structure.
Shortly after initial crystallisation, and still at very high
residual magmatic temperatures, BAOC was obducted over
the Ossa-Morena Zone along a presently not observable
shear zone. Evidence for this obduction exists as small
internal sub-horizontal, north-verging shears and as a recrystalisation anisotropic fabric preserved in the lower
gabbroic sequence and carried by the very high temperature
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
metamorphic paragenesis. Normal regional metamorphism
may be ruled out as the origin for this metamorphic texture,
because no evidence for it is ever found above the lower
gabbroic section; a similar absence in the underlying peridotites, where the anisotropic stresses induced by obduction
at the base of the ophiolitic nappe should be at their
maximum, is easily explained by later serpentinisation
obliteration. Further record of this obduction-induced recrystallisation is found in the compositional «mismatch»
between Cr-spinel and olivine, the latter being too forsteritic
for the observed composition of the spinel, if equilibrium
co-crystallisation of both at normal magmatic temperatures
is assumed. Chemical equilibrium at the actual chemical
compositions of both minerals can be achieved at significantly lower temperatures, but no other mineralogical evidence for such lower crystallisation temperatures exists and
thus the present compositions must record a subsolidus
reequilibration of Cr-spinel and olivine.
Obduction also lead to a geologically sudden chilling of
BAOC. Mineralogical evidence for this is the lack of further
primary reequilibration of spinel, whose composition indicates a high closure temperature for the subsolidus reactions
with olivine, and the lack of exsolution textures within
ilmenite, which are to be expected if ilmenite with the
observed composition were slowly cooled from normal
basic magma temperatures. The hematite peaks systematically observed in X-ray powder diffractograms cannot be
taken as evidence for crypto-exsolution textures within
ilmenite, because of the simultaneous presence of petrographically invisible rutile and pseudorutile, which indicates decomposition of ilmenite in supergene conditions.
Modelling (see appendix for details), assuming conduction
as the only means of heat transfer, of the thermal evolution
of BAOC after obduction, shows that the obducted hot slab
cools down in a few hundred thousand years and causes
negligible heating of the overthrusted continental crust (Figs
7 and 8). The results obtained by running such simple model
must be regarded as conservative, because no attempt is
made to model possible and far more efficient mechanisms
of heat transfer involving fluid circulation, which is still
poorly understood at the ductile regimes suggested by the
high temperatures needed to account for the metamorphic
paragenesis. It seems thus improbable that BAOC obduction
is recorded as a thermal metamorphic event in the rocks of
the autochthonous Ossa-Morena Zone or that heat transfer
from BAOC caused the fluid releases (from decomposition
of the underlying carbonate platform, for instance), whose
circulation is attested by the later intense metasomatism
associated to BAOC internal shear zones. The model also
excludes the possibility that the observed greenschistamphibolite facies is a record of the late stages of the
obduction event.
The greenschist-amphibolite metamorphism is a regional
event that affects BAOC and most of the geological units
surrounding it, BIC intrusives being the outstanding exception. By the time of this metamorphic event, the deforma-
57
tion style of BAOC had changed substantially: instead of
north verging internal shear zones, BAOC now shows very
important WNW-ESE left-lateral shears with almost no
vertical movement (D2 deformation phase), and displays, at
the gabbroic rocks level, a prominent stretching lineation
materialised by the metamorphic newly formed green
hornblende-actinolite crystals. There is also some evidence
[13,2] for west thrusting along N-S shear zones, which is
geometrically compatible with the stress field that originated the WNW-ESE shears and with the linear fabric
displayed by the gabbroic rocks. As the detailed study of the
main shear zones shows, this tectono-metamorphic regime
gradually changed with time. Gradually decreasing temperatures are recorded (mineralogically and texturally)
within the alteration zones lining up the shear zones and, at
the same time, the shears gradually entered the brittle
deformation regime and acquired an ever increasing SWverging thrust component. The final stage of this varying
tectono-metamorphic process may be described as a greenschist or lower facies metamorphism accompanying brittle
SW-verging thrusts along the earlier strike-slip shear zones
(D3 deformation phase), themselves object of intense carbonate (mainly calcitic) metasomatism.
The geometrical compatibility between the D2 linear
fabric of BAOC and the primary fabrics of the primitive
gabbros of BIC deserves attention and is a clear indication
of a non-synchronous origin for both igneous bodies. As a
matter of fact, the fabrics are carried in BAOC by a
metamorphic
paragenesis
indicating
low
amphibolite/greenschist facies temperatures, and in BIC by
an igneous paragenesis indicating normal primitive gabbro
crystallisation temperatures. The simpler interpretation
seems to be a postulated synchrony between the D2 defor
mation phase of BAOC and the initial stages of BIC
intrusion. BIC would thus be the intrusives related to the
continental collision recorded in this segment of the
Variscan orogen. This is coherent with the observation that
D3 structures, and its associated metamorphism, as revealed
in BAOC shear zones, are observed in very large regions of
both the Ossa-Morena and the South Portuguese zones in a
seemingly deformational and metamorphic continuum.
It is worth noting that, although three deformation phases
have been described for BAOC (e.g. [13,2]), all of them
generated macrostructures whose orientations are compatible with a general NE-SW compression typical of the
Variscan Orogen in South Portugal and Southwest Spain.
The whole of BAOC evolution can thus be envisaged as a
single geodynamic cycle of continental approach and collision, wherein obduction is just a minor event in the period
preceding climactic deformation and metamorphism; the
present geological record documents mainly the waning
stages following that paroxysmal phase. Also worth mentioning is the fact that, at least in its portuguese tract, BAOC
evolved in a dry environment until metamorphic degassing
caused the influx of water and, later, of water-CO2 fluids. In
fact, significant amounts of water did not enter BAOC
58
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
Fig. 7. Numerical modelling of the post-obduction thermal evolution of BAOC and underlying rocks, considering a normal geothermal gradient within BAOC
and a basal temperature of 920ºC. A – Geotherms plotted every 10000 years between 0 and 100000 years. B – Idem, plotted every 100000 years, between
0 and 106 years. C – Modelled thermal evolution of the carbonate sequence immediately underlying BAOC; T-t lines are drawn every 0.1 km for the first
10000 years after obduction. D – Idem for the first 0.1 My.
before extensive peridotite serpentinisation occurring during the initial stages of WNW-ESE shear zone development.
Prior obduction-related recrystallisation of gabbros was
anhydrous and their subsequent amphibolite-facies metamorphism was far from complete. Since oceanic serpentini-
sation did not occur prior to obduction, this may be taken as
yet another indication that BAOC was composed of very
young rocks at the time of obduction or that ocean water
circulation was restricted to levels above the present erosion
surface.
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
59
Fig. 8. Numerical modelling of the post-obduction thermal evolution of BAOC and underlying rocks, considering a basal slab of uniform temperature equal
to 920ºC within BAOC. A – Geotherms plotted every 10000 years between 0 and 100000 years. B – Idem, plotted every 100000 years, between 0 and 106
years. C – Modelled thermal evolution of the carbonate sequence immediately underlying BAOC; T-t lines are drawn every 0.1 km for the first 100000 years
after obduction. D – Idem for the first 1 My.
. Appendix 1
The modelling of the thermal evolution of the crustal
section after BAOC emplacement was treated as a one
dimensional problem where a sequence of oceanic crustal
rocks of 5 km thick is put upon a stretched crustal section
some 20 km thick. The base of the thrust is set to 800 to 900
ºC range in agreement with field data on the temperature of
60
J. Figueiras et al. / Geodinamica Acta 15 (2002) 45–61
Table A1
Variable definitions.
Variable
Definition
Units (SI)
A
C
hr
hs
k
qm
T
Ts
t
z
j
ρ
Volumetric heat production
Heat capacity
Characteristic distance
Mass heat production in the crust
Thermal conductivity
Conductive heat flux
Temperature
Surface temperature
Time
Depth
Thermal diffusivity
Density
W m–3
J kg–1 K–1
m
W kg–1
W m–1 K–1
W m–2
K
K
s
m
m2 s–1
Kg m–3
thrust emplacement. Thermal gradients for both crustal
blocks were calculated according to the equation [2] (see
Table A1 for the variable definitions):
2
qm z qHs h r
− z/h
T = Ts +
+
共1 − e r兲
k
k
The heat flux was adjusted to obtain a basal temperature
of BAOC of 920º C, and an approximate geothermal gradient in the range 10-20ºC.Km–1 for the autochthon crustal
rocks. The mean thermal conductivity of the continental
crust and ophiolite was calculated using data given for the
considered rock types in Appendix 2 of Turcotte and
Schubert [29], and weighted according to the volumes of
each rock type in the crustal block. Mean densities were
calculated in the same way for both blocks and the values
used for the mass heat production due to radioactive
elements (Hs) were calculated as a function of their mean
concentration in the upper continental crust and oceanic
crust. The characteristic distance used for both blocks was
10 Km in agreement with the general knowledge on
radioactive element concentration in the crust. Surface
temperature was arbitrarily set to zero. Both these data
values (densities and radioactive heat production) were
consulted in Carmichael [30].
The initial conditions assume an instantaneous thickening of the crust with no heat lost. The model calculates the
variation of the temperature of the crustal section with time,
in one-dimension by solving numerically the heat conduction equation
2
⭸T = j⭸ T + A
2
⭸t
qC
⭸z
The boundary conditions for the base of the crustal
section admits a constant heat flux of 0.03 W.m–2, which
sets a constant thermal gradient of nearly 13 ºC/Km at the
base of the crust, according to the equation (Fourier law)
qm = kdT
dz
The top of the crustal section is kept at the constant
temperature of 0ºC. The model does not calculate any heat
loss mechanism other than conduction. This rules out the
possibility of convective heat loss as well rendering the
calculations rather conservative. However, the high temperatures involved suggest a predominant ductile regime
and fluid circulation in such conditions is very ill-known, so
it has been discarded for this reason. The involvement of
fluids is expected to be more important in the later cooling
stages of the crustal section. Two sets of runs were performed for a maximum period of 1 Ma, where different
temperature profiles were assumed for the BAOC section: i)
a normal gradient from surface to the base of the oceanic
crust at 920º C; ii) the same gradient but keeping the lower
2 Km of the oceanic crust at 920º C, just as a sub-solidus hot
magmatic chamber.
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