Metamorfismo Varisco em rochas ígneas do Câmbrico Inferior

Metamorfismo Varisco em rochas ígneas do Câmbrico Inferior
(Maciço de Évora): Novos dados geocronológicos U-Pb SHRIMP em
zircões dos ortognaisses das Alcáçovas
Variscan metamorphism overprint on Early Cambrian igneous rocks (Évora Massif): New UPb SHRIMP zircon geochronology from the Alcáçovas orthogneisses
Pereira, M.F. (1), Chichorro, M. (1), Williams, I.S., (2), Silva, J.B., (3)
(1) Departamento de Geociências, Centro de Geofísica de Évora, Universidade de Évora,
Apt.94, 7001-554 Évora, Portugal
(2) Geochronology and Isotope Geochemistry, Research School of Earth Sciences, Australian
National University, Canberra, Australia
(3) Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Edifício C3,
Campo Grande, Lisboa, Portugal
E-mail (s): mpereira@uevora.pt,
SUMÁRIO
Dados novos de idades obtidas por U-Pb SHRIMP em zircão dos ortognaisses das Alcáçovas indicam que o seu
protólito é de idade Câmbrica (526.5±9.9 Ma). Zircão resultante de processos de recristalização no estado sólido
e também de novos crescimentos associados a condições de fusão parcial, foram por sua vez datados do Viseano
(330-342 Ma), confirmando que a deformação que afecta estas rochas está relacionada com condições de
metamorfismo na fácies anfibolítica que preservam.
Palavras-chave: Zona de Ossa-Morena, magmatismo Câmbrico, metamorfismo Carbónico, SHRIMP
SUMMARY
New U-Pb SHRIMP zircon ages from the Alcaçovas orthogneisses demonstrate that their protoliths were
Cambrian igneous (526±9.9 Ma). Rims of solid-state recrystallized zircon together with metamorphic
overgrowths were dated at Visean (339.7±5.5 Ma) indicating that these rocks were deformed under amphibolite
facies metamorphic conditions.
Key-words: Ossa-Morena Zone, Cambrian magmatism, Carboniferous metamorphism, SHRIMP
Introduction and geological setting
In Portugal, despite of the insufficiency of available
radiometric data on pre-Variscan rift-related
magmatism in the Ossa-Morena Zone, the obtained
results were interpreted to be representative of an
important Ordovician plutonic event [1-3]. This
thought was supported by regional correlations with
other apparently similar rocks dated in Spain [1,4].
Recently, the application of U-Pb SHRIMP zircon
dating revealed the development of a rift-related
magmatism of Cambrian age with bimodal alkaline
and calc-alkaline signature (at ca. 530-500 Ma),
proceeds by alkaline-peralkaline Ordovician
magmatism (at ca. 490-460 Ma) [e.g. 5,6,7].
In the Évora Massif (westernmost domains of the
Ossa-Morena Zone) [8-9], the Alcáçovas
orthogneisses were interpreted to represent Early-
Middle Ordovician intrusions, on the basis of wholerock Rb-Sr isotopic geochronology (456±23 Ma,
with initial 87Sr/86Sr=0.71±0.0001, MSWD=0.65)
[3]. The same authors admitted that these rocks were
affected by Devonian deformation and later
overprinted by Carboniferous metamorphism, as
pointed by Rb-Sr biotite/whole-rock and K-Ar
biotite average ages of 333 Ma. In the same study,
unmetamorphosed dykes of felsic porphyries
intrusive on the orthogneisses yielded an age of
319±5 Ma with initial 87Sr/86Sr=0.708±0.0003
(MSWD=0.14). They were used to define the upper
limit of the deformation and metamorphism that
affected the Alcáçovas orthogneisses.
Our work presents new U-Pb SHRIMP zircon age
determinations on the Alcáçovas orthogneisses that
were performed to test the previously obtained
93
geochronology results that used Rb-Sr whole-rock,
Rb-Sr biotite/whole-rock and K-Ar biotite dating.
New U-Pb SHRIMP zircon ages for the Alcáçovas
orthogneiss did not provide an unequivocal age for
the intrusion (with the best approximate age at ~540
Ma) and a metamorphic overprint at ~370 Ma [10].
Analytical procedures
A felsic orthogneiss deformed under amphibolite
facies metamorphism was sample in the road
Montemor-o-Novo - Alcáçovas (ALC-10).
Zircon grains were separated from sample ALC-10
after crushing, using heavy liquids separation, by
sieving and magnetic separation at the Laboratory of
the Departamento de Geociências (Universidade de
Évora). At the Research School of Earth Sciences
(Australia National University, Canberra) the
extracted grains were selected and hand-picked
according to colour, size and morphology and then,
mounted in a synthetic resin and polished. The
mount was photographed in transmitted and
reflected light using a Scanning Electronic
Microscope and cathodoluminescence imaging. The
selected zircons were then analysed using SHRIMP.
A focused primary beam of heavy charged ions O2was used to sputter atoms from selected zircon
surface areas. The obtained a spot size hand a
diameter of 10-20 μm, and measurements of U-ThPb isotopes were analysed by mass spectrometry.
Figure 1 - Concordia Tera-Wasserburg diagram
from sample ALC-10 (Alcáçovas orthogneiss).
Note: The older exotic core with a Paleoproterozoic
age of 2.0 Ga is not represented.
Five analyses represent zircon growth on a
magmatic stage. Three spots point for Lower
Cambrian ages (521-529 Ma) (Fig.2) and two spots
gave younger ages (crystal 7: 500 Ma and crystal 8:
497Ma). We consider the last two as partly reset
ages probably due to lead-loss related with
Carboniferous metamorphism. Four spots are related
with Upper Paleozoic (Visean age) recrystalization
fronts (crystal 1, 3 and 5: 330-342 Ma). The spot
from crystal 4 on a metamorphic overgrowth gave
the same age (338 Ma) (Fig. 3). The remaining spot
(crystal 9) was done on an older exotic core with a
Paleoproterozoic age of 2.0 Ga.
SHRIMP results
Zircons separated from sample ALC-10, have smalldimensions (<250μm), are mostly slightly pink to
colourless, subeuedric, and in some cases present
modified subrounded faces. They are in general
prismatic with relatively well developed two
pyramid and prisms, with (211) and (100) crystal
faces, but there also exist acicular crystals. The
majority of crystals present a concentric zoning but,
in some cases the zoning is not obvious. They show
inclusions of large axial cavities and canalicules
(melt inclusions?) and mineral dark phases
(biotite?). The cathodoluminiscence imaging was
useful to observe that igneous zonation is affected
by many unzoned transgressive embayments, mainly
in the external part (crystals 3, 2 and 1), but also
deep in zircon structure (crystal 10). The observed
cores are frequently subrounded (crystal 8) and
sometimes are difficult to distinguish between
recrystallization fronts and new growths (crystal 10).
Finally, some zircons present typical pyramidal new
metamorphic additions (crystal 4).
Eleven SHRIMP analysis were done and the results
are plotted in a concordia Tera-Wasserburg diagram
(Fig.1).
Figure
2
–
Representative
SEM
and
cathodoluminiscence imaging of selected Cambrian
zircons from sample ALC-10.
94
sillimanite, biotite, feldspar and quartz that defines
the foliation of the Alcáçovas orthogneisses. The
transgressive mantles and/or isolated patches of
recrystallization in zircons, typical on these studied
rocks, are a common feature of high-grade
metamorphic rocks. These features can be related
with to thermal activated solid-state intracrystalline
deformation mechanisms that contributed to reduce
the lattice strain [15]. Their characteristic low Th/U
signature (totally different from the igneous zircons)
together with the low spread in concordant U-Pb
ages, suggest that they didn’t retain a chemical
“igneous memory”. This was probably due to the
effect of an extended period of high-grade
conditions. The presence of a magmatic core with a
new addition of zircon suggests that the
orthogneisses have experimented melting conditions.
Finally, the oldest measured age obtained from
crystal 9 (2.0 Ga- Paleoproterozoic) is indicative of
crustal
contamination
during
the
magma
emplacement. Similar ages are found in the Serie
Negra Ediacaran sediments [16] which represent the
host rock of this Early Cambrian magmatism.
Figure
3
–
Representative
SEM
and
cathodoluminiscence images of selected Visean
trangressive recrystalization zircon fronts (crystals 3,
1 and 2) from sample ALC-10. Note: A 50μm new
addition of zircon with (211) crystal faces formed as
result of partial melting conditions (crystal 4).
Acknowledgements
This work is a contribution for the IGCP Project
497.
Discussion
Only three spot analyses on individual clearly
magmatic zircons from sample ALC-10 are
statistically related but we consider them significant
enough to define the age of magmatism at 526.5±9.9
Ma (Lower Cambrian following the ISC [11]). This
result invalidates the previous interpretation of an
Ordovician age, based on Rb-Sr isotope
geochronology.
The U-Pb SHRIMP zircon ages from the Alcáçovas
orthogneisses are comparable with those obtained by
the application of the same method on similar OssaMorena Zone rocks from Spain [9]. This Cambrian
thermal event has been tentatively associated to
post-collisional Cadomian processes [12,10]. This
geodynamic stage represents the transition from the
Cadomian accretion evolution to an Early Paleozoic
extensional-dominated regime [13-14]. This period
of continental crustal growth is characterized by the
emplacement of calc-alkaline melts, generated due
to decompressional melting after thickening of the
continental crust [5] and/or derived from the
underlying contaminated mantle.
Recrystallized zircons together with the new zircon
growth (all with Th/U ratio lower than 0.04) in
sample ALC-10 span the age range 330-342 Ma
from five spots. These results gave a statistically
significant estimate age of 339.7±5.5 Ma (Visean
following the ISC [11]). The radiometric results of
K-Ar biotite ages of ca. 333 Ma considered to be
related to a metamorphism much younger than
deformation [3] is not confirmed by our data. If fact,
the zircon age forming and recrystallization event
seems to be linked to the growth of aligned
References
[1] Lancelot,
J.R.,
Allegret,
A., 1982.
Radiochronologie U/Pb de l’orthogneiss alcalin de
Pedroso (Alto Alentejo, Portugal) et evolution antehercynienne de l’Europe occidentale. Neues Jahrb.
Mineral. Monash.: 385-394.
[2] Priem, H.N.A., Boelrijk, N.A.I.M., Verschure,
R.H., Hebeda, E.H., Verdurmen, E.A.Th., 1970.
Dating events of acid plutonism through the
Paleozoic of the Western Iberian Peninsula. Eclogae
Geol. Helv., 63: 255-274
[3] Priem, H.N.A, Boelrijk, N.A.I.M., Hebeda, E.H.,
Schermerhorn, L.J.G. 1986. Isotopic ages of the
Alcáçovas orthogneiss and the Beja Porphyries,
South Portugal. Comunicações dos Serviços
Geológicos de Portugal 72, 3-7.
[4] Garcia Casquero, J.L., Boelrijk, N.A.I.M.,
Chacón, J., Priem, H.N.A, 1985. Rb-Sr evidence for
the presence of Ordovician granites in the deformed
basement of the Badajoz-Cordoba belt, SW Spain.
Geol. Rundschau, 74: 379-384.
[5] Ordoñez-Casado, B., 1998. Geochronological
studies of the Pre-Mesozoic basement of the Iberian
Massif: the Ossa-Morena Zone and the
Allochthonous Complexes within the Central Iberian
Zone. Diss. ETH Nº.12940, 1-235
[6] Sanchez-Garcia, T., Bellindo, F., Quesada, C.,
2003. Geodynamic setting and geochemical
signatures of Cambrian-Ordovician rift-related
igneous rocks (Ossa-Morena Zone, SW Ibéria).
Tectonophysics 365: 233-255.
95
[7] Ribeiro, M., Mata, J., e Munhá, J. 1992.
Magmatismo do Paleozóico Inferior em Portugal. In
Gutierrez-Marco, J. C., Saavedra, J. & Rábano, I.
(Eds.). Paleozoico Inferior de Ibero-América. Coord.
M. J. Liso Rubio, Universidad de Extremadura, pp.
377-395.
[8] Carvalhosa, A., 1983. Esquema geológico do
Maciço de Évora. Comunicações dos Serviços
Geológicos de Portugal 69, 201-208.
[9] Pereira, M.F., Silva, J.B., Chichorro, M., 2003.
Internal structure of the Évora Massif: The Évora
High-grade metamorphic terrains and the
Montemor-o-Novo Shear Zone (Ossa-Morena Zone,
Portugal). Geogaceta 33, 71-74.
[10] Cordani, U.G., Nutman, A.P., Andrade, A.S.,
Santos, J.F., Azevedo, M.R., Mendes, M.H., Pinto,
M.S. 2006. New U-Pb SHRIMP zircon ages for prévariscan orthogneisses from Portugal and their
bearing on the evolution of the Ossa-Morena
Tectonic Zone. Anais da Acad. Brasileira de
Ciencias 78(1): 133-149
[11] Gradstein, F.M., Ogg, J.G., Smith, A.G.,
Bleeker, W., Lourens, L.J., 2004. A new Geological
Time Scale with special reference to Precambrian
and Neogene. Episodes 27 (82), 83-100
[12] Eguiluz, L., Gil-Ibarguchi, J.I., Abalos, B.,
Apraiz, A., 2000, Superposed Hercynian and
Cadomian orogenic cycles in the Ossa-Morena zone
and related areas of the Iberian Massif. GSA
Bulletin 112, 9, p. 1398-1413.
[13] Silva, J.B., Pereira, M. F., 2004, Transcurrent
continental tectonics model for the Ossa-Morena
Zone Neoproterozoic-Paleozoic evolution (SW
Iberian Massif, Portugal). International Journal of
Earth Sciences, 93, p. 886-896.
[14] Pereira, M.F., Chichorro, M., Linnemann, U.,
Eguiluz, L., Silva, J.B., 2006, Inherited arc signature
in Ediacaran and Early Cambrian basins of the OssaMorena Zone (Iberian Massif, Portugal):
paleographic link with European and North African
Cadomian correlatives. Precambrian Research 144:
297-315
[15] Hoskin, P.W.O. and Black, P.L., 2000.
Metamorphic zircon formation by solid-state
recrystallization of protolith igneous zircon, J.
metamorphic Geol., 18, 423-439
[16] Chichorro, M., Pereira, M.F., Williams, I.,
Silva, J.B., this volume, Clues for Cadomian
orogenic events in SW Iberian Massif: U/Pb –
SHRIMP zircon evidence from the Ediacaran Serie
Negra sediments
(Escoural Formation, OssaMorena Zone, Portugal).
96