Magmatic evolution and emplacement of the Paleoproterozoic

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INTERNATIONAL CONFERENCE ON A-TYPE GRANITES AND RELATED ROCKS THROUGH TIME
Magmatic evolution and emplacement of the Paleoproterozoic, anorogenic
oxidized A-Type Redenção Granite, eastern Amazonian craton, Brazil
D. C. Oliveira 1, R. Dall’Agnol 2, S. P. Neves 3, R. I. F. Trindade 4, M. A. Oliveira 1, G.
Mariano 3, P. B. Corrêia 3
1 Faculty of Geology, Federal University of Pará, Campus of Marabá, [email protected]
2 Institute of Geosciences, Federal University of Pará, [email protected]
3 Department of Geology, Federal University of Pernambuco, [email protected]
4 Department of Geophysics, University of São Paulo, [email protected]
Introduction and Field Relationships
The Redenção pluton is part of the 1.88 Ga, anorogenic, A-type Jamon suite that intruded 2.97 –
2.86 Ga-old Archean granitoids of the Rio Maria Granite-Greenstone Terraine in the eastern
Amazonian craton (northern Brazil). The Redenção and other plutons of the Jamon Suite are
generally unfoliated and, except for local magmatic foliation developed on the border,
deformational structures are restricted to fracturing and faulting. Gravity survey shows that the
Redenção pluton is a 6 km-thick, sheeted-like intrusion emplaced at shallow crustal level, where
it cross-cuts the E-W to NW-SE structural trends of the Archean granitic basement. Contacts are
strikingly discordant and sharp. Angular xenoliths of the Arco Verde tonalite are commonly
observed near the border of the pluton, indicating a high contrast in viscosity between the
granitic magma and the Archean bedrock. Swarms of mafic, intermediate, and felsic dykes are
associated with the Jamon Suite. A composite mafic-felsic dike cutting Archean rocks was
described (Dall’Agnol et al. 2005). The felsic dikes yielded Pb-Pb zircon ages of 18854 Ma and
are coeval with the granitic plutons, demonstrating that the latter were emplaced in an
extensional tectonic regime.
Petrography and Magmatic Evolution
The Redenção pluton is formed essentially of monzogranites disposed in near-concentric zones
(Fig. 1). The less evolved facies is an even-grained, coarse biotite-hornblende monzogranite,
locally enriched in cumulatic amphibole ± clinopyroxene, which occurs in the southern part of
the pluton. It grades to a coarse, even-grained (hornblende)-biotite monzogranite. Evolved
leucogranites occur in the central part of the body. Seriated and porphyritic biotite monzogranite
facies intrusive in the coarse- even-grained (hornblende)-biotite monzogranite configure annular
structures in the central and southern areas of the pluton. The magmatic zoning is marked by a
systematic decrease of mafic mineral modal content, plagioclase/potassium feldspar,
amphibole/biotite and anorthite content of plagioclase. TiO2, MgO, FeOt, CaO, P2O5, Ba, Sr, and
Zr decrease, and SiO2, K2O, and Rb increase in the same way. Magmatic differentiation was
controlled by fractionation of early crystallized phases, including amphibole±clinopyroxene,
andesine to calcic oligoclase, ilmenite, magnetite, apatite, and zircon (Oliveira et al. 2009). Mass
balance calculations indicate that the clinopyroxene-amphibole-biotite monzogranite was not
able to generate the amphibole-biotite monzogranite by fractional crystallization processes.
Geochemical modeling suggests, on the other hand, that the clinopyroxene-bearing less evolved
rocks were formed by accumulation of mafic phases from the amphibole-biotite monzogranite.
Mass balance calculations suggest additionally that the biotite-hornblende monzogranite and
biotite monzogranites were probably linked by fractional crystallization processes. The
leucogranites, on the other hand, do not follow the general geochemical trends defined for the
other varieties and are most probably derived from an independent magma, as indicated by the
geochemical modeling. Finally, the geochemical characteristics of the porphyritic facies suggest
that at least part of this variety could represent the hybrid product of mingling processes
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University of Helsinki, Finland
INTERNATIONAL CONFERENCE ON A-TYPE GRANITES AND RELATED ROCKS THROUGH TIME
involving coarse biotite monzogranites and leucogranites. Processes of mixtures of magmas have
also been proposed as responsible for the magmatic evolution of the Wangrah A-type granites of
Australia (Klimm et al. 2008) and are probably more common in granitic plutons than generally
admitted.
Anisotropy of Magnetic Susceptibility Study
Magnetic Mineralogy, Magnetic Susceptibility and Zonation
In the Redenção pluton, modal contents of Fe-Ti oxide minerals vary between 0.5 and 2%, and
magnetite is dominant over ilmenite. This granite is thus a typical magnetite-series granite
(Ishihara 1981). The mean bulk susceptibility, K [(k1 + k2 + k3)/3], varies between 1.05x10-3 and
54.72x10-3 SI with an average of 11.55x10-3 SI. The K values typically increase for higher
opaque contents and decreases with quartz and K-feldspar contents. The highest K values are
concentrated in the southern part of the pluton and decrease towards the center of the intrusion,
the lowest K values being found in the leucogranites, where mafic modal contents are the lowest
and magnetite martitization is more intense. This picture is consistent with the geochemical
zoning. Thermomagnetic curves confirm the magnetic mineralogy deduced from petrographic
observations and are consistent with the high magnetic susceptibilities observed in different
facies of the Redenção granite (Oliveira et al. Submitted).
Magnetic Fabric
In the Redenção Granite, the AMS fabric is controlled mostly by magnetite, which is an earlycrystallized accessory mineral. In rocks in which K is carried dominantly by ferromagnetic
minerals the AMS probably results from the shape anisotropy of magnetite grains (e.g., Archanjo
et al. 1995; Ferré et al. 1999). This is attested by AAR measurements that have principal axes
coaxial with the AMS ellipsoids for most of the analyzed samples (Oliveira et al. submitted).
These AAR ellipsoids correspond to the low-coercivity window of 0 to 50 mT, being controlled
only by the coarse-grained multidomain magnetite grains. The anisotropy degree in the
Redenção pluton is low, with most sites presenting values below 1.10, and does not correlate
with petrographic domains. Sites with P > 1.10 are only found at the southwestern border of the
pluton. The preponderance of low P values is consistent with petrographic observations showing
a lack of solid-state deformation in the constituent minerals of the different petrographic facies.
The shape of the AMS ellipsoids varies widely within the pluton. The site mean shape parameter
for all facies ranges from -0.93 to 0.93, with oblate ellipsoids (T > 0) being slightly more
abundant than prolate ones (Oliveira et al. submitted). Contrary to the other facies where no clear
tendency in the ellipsoid shape can be defined, seriated biotite-monzogranites and porphyritic
biotite-monzogranites show a strong prevalence of oblate fabrics with average T values of 0.39
and 0.12, respectively.
The orientation of magnetic foliations and lineations are shown in Figure 1. Magnetic foliations
and lineations of coarse biotite monzogranite and hornblende-biotite monzogranite show a clear
concentric pattern in map view (Oliveira et al. submitted). All lineations in these facies have
gentle plunges (<30°). Gently dipping foliations, with dips below 40°, are also dominant in all
sectors where these facies occur, except for the northeastern sector where foliations dip steeply
and show consistent NW-strikes parallel to the contact with the host-rocks. Seriated and
porphyritic biotite monzogranites, which crop-out forming annular structures, also show a
concentric pattern of magnetic foliations that systematically dip outwards with shallow to
moderate plunges. Magnetic lineations in the seriated biotite monzogranite have shallow plunges
towards NW or SE. Magnetic lineations in the porphyritic biotite monzogranite plunge gently,
outward of the ring-pattern distribution of this facies. In contrast to the other facies, magnetic
foliations in the central leucogranite show dominantly steep dips. They form a ring pattern in its
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University of Helsinki, Finland
INTERNATIONAL CONFERENCE ON A-TYPE GRANITES AND RELATED ROCKS THROUGH TIME
northeast contact with the coarse-grained biotite monzogranite. Magnetic lineations in this facies
trend dominantly NW-SE, showing steep to moderate plunges in the northeast sector of the
pluton and gentle plunges elsewhere.
Emplacement Model
A model of emplacement for the Redenção pluton must account for its tabular shape, the
dominantly shallowly dipping foliations and lineations, and the concentric arrangement of
fabrics and petrographic facies (Oliveira et al. submitted). We propose an intrusion model in
which the Redenção pluton formed by the amalgamation of sills emplaced through three major
pulses (Fig. 2): (1) the emplacement of Redenção started with the coarse-grained (hornblende)biotite monzogranite (Fig. 2a). The feeding zone for this first magma batch was likely placed in
the northeastern domain, the thickest sector of the pluton, where magnetic foliations and
lineations are steep. The vertical fabrics observed over the thickest sector of the pluton mark the
ascent of magma through of dykes. The magma would then spread horizontally, diverging
towards south and west (Fig. 2a); (2) the second batch of magma corresponds to the seriated and
porphyritic monzogranites (Fig. 2b), which were probably emplaced through a succession of sills
when the previous magma batch still had a significant proportion of melt. Some of these sills had
a leucogranite composition and mingled with the biotite ± hornblende monzogranite magma; (3)
in the third stage (Fig. 2c), emplacement of the late leucogranite facies into the hot previously
emplaced granites resulted in in situ inflation of the magma chamber and local doming. Contacts
between the leucogranite facies and the host hornblende-biotite monzogranites are usually sharp
and its magnetic fabric pattern contrasts with that of previously emplaced types. This suggests
that this last magma batch encroached on an almost completely crystallized magma body,
pushing the horizontal sills upward in the center of the intrusion and giving raise to the
concentric pattern of facies in map view. Deformation induced by the emplacement of the
leucogranite stock would also rotate lineations and foliations at the border of the pluton giving
rise to the concentric pattern of the magnetic fabric and to the local vertical foliations at the
contact with host rocks.
Conclusions
At the end of the Paleoproterozoic (~1.88 Ga), the Carajás province recorded a tectonothermal
event that resulted in the emplacement of several anorogenic, A-type granites. These plutons
were emplaced in an extensional tectonic setting coeval with WNW-ESE- to NNW-SSE-striking
diabase and granite porphyry dikes. The magmatic evolution of the Redenção pluton is relatively
complex involving several distinct processes, with mingling relationships that indicates multiple
magma injections in its construction. The magnetic fabric study of the Redenção pluton, together
with petrographic and geochemical data, shows that its anisotropy of magnetic susceptibility is
carried by early-crystallized coarse-grained magnetite. The concentric orientation of gently
dipping magnetic foliations and shallow to moderately plunging magnetic lineations in the
different facies of the pluton are interpreted as resulting from a laccolithic emplacement where
the even-grained monzogranite and the seriated and porphyritic granites intruded as sills; the
final shape of the pluton being acquired after the intrusion and inflation of the central
leucogranite.
References
Archanjo, C.J., Launeau, P., Bouchez, J.L., 1995. Magnetic fabric vs. magnetite and biotite shape fabrics of the
magnetite-bearing granite pluton of Gameleiras (northeast Brazil). Phys. Earth Planet. Interiors. 89, 63–75.
Dall’Agnol, R., Teixeira, N.P., Rämö, O.T., Moura, C.A.V., Macambira, M.J.B., Oliveira, D.C., 2005. Petrogenesis
of the Paleoproterozoic, rapakivi, A-type granites of the Archean Carajás Metallogenic Province, Brazil.
Lithos 80, 101-129.
Ferré, E.C., Wilson, J. and Gleizes, G., 1999. Magnetic susceptibility and AMS of the Bushveld alkaline granites,
South Africa. Tectonophysics, 307: 113-133.
August 14-20, 2010
University of Helsinki, Finland
INTERNATIONAL CONFERENCE ON A-TYPE GRANITES AND RELATED ROCKS THROUGH TIME
Klimm, K., Holtz, F., King, P.L., 2008. Fractionation vs. magma mixing in the Wangrah Suite A-type granites,
Lachlan Fold Belt, Australia: Experimental constraints. Lithos 102, 415-434.
Oliveira, D.C., Neves, S. P., Trindade, R. I., Dall’Agnol, R., Mariano, G., Corrêa, P. B. Submitted. Magnetic
anisotropy of the Redenção Granite, eastern Amazonian craton (Brazil): Implications for the emplacement of
A-type plutons. Tectonophysics.
Oliveira, D.C., Dall’Agnol, R., Barros, C.E.M., Oliveira, M.A., 2009. Geology, geochemistry and magmatic
evolution of the Paleoproterozoic, anorogenic oxidized A-type Redenção granite of the Jamon Suite, eastern
Amazonian Craton, Brazil. Canadian Mineralogist 47 (6), 1441 – 1468.
Figure 1 - Geological map of the Redenção pluton showing the areal distribution of granitic facies and AMS
directional data (modified from Oliveira et al., 2009). (a) Magnetic foliations (normal to K3) and (b) Magnetic
lineations (parallel to K1).
Figure 2. Emplacement model for the Redenção granite
(left panel: map view, right panel: vertical sections). (a)
Emplacement of the first magma batch of (hornblende)biotite monzogranite as a horizontal sheet (sill). We
hypothesize that the feeding dykes were oriented
parallel to the coeval dykes observed in country rocks
and were placed in the northeastern sector of the pluton,
where gravity data points to a thickest root.
Geochemical and petrographic zoning follows the
direction of magma flow from northeast to the south and
west. (b) Emplacement of the second batch of magma
through a series of sills into the still molten
(hornblende)-biotite monzogranite. (c) Emplacement of
the last magma batch of leucogranite, forming a circular
body in the core of the pluton. This last magma pulse
has deformed the previously emplaced sills, pushing the
horizontal layers upward in the center of the pluton and
giving rise to the concentric pattern of petrographic
facies observed in map view.
August 14-20, 2010
University of Helsinki, Finland
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