Anisotropy of Magnetic Susceptibility of the Redenção Granite, Eastern Amazonian
Craton: Implications for the Emplacement of a Paleoproterozoic Anorogenic A-type
Suite
Davis Carvalho de Oliveira 1, Sérgio Pacheco Neves2, Roberto Dall’Agnol1 , Gorki Mariano2,
Paulo B. Correia2
1Group
of Research on Granite Petrology, Centro de Geociências, Universidade Federal do Pará, Caixa postal 1611, 66075100 Belém, Pará, Brazil. E-mail address: davis@ufpa.br
2Departament of Geology, Universidade Federal de Pernambuco, 50740-530 Recife, Brazil
Introduction
The eastern Central Amazonian Province (Amazonian Craton) is divided into two
Archean tectonic domains, the 3.0-2.86 Ga Rio Maria Granite-Greenstone Terrane
(Macambira and Lafon 1995, Dall’Agnol et al. 1997) and the rift-related Carajás Basin,
dominantly composed of 2.76-2.55 Ga metavolcanic rocks, banded iron formations, and
granitoids. The Rio Maria Granite-Greenstone Terrane is limited to the east by the
Neoproterozoic Araguaia Belt, whose evolution is associated with the tectonic and
metamorphic Brasiliano (Pan-African) cycle that did not significantly affect the Amazonian
Craton. The eastern part of the Amazonian Craton remained stable until 1.88 Ga when an
episode of distension and underplating led to the generation and emplacement of A-type
granites (Jamon Suite) and associated mafic and felsic dikes in the Rio Maria GraniteGreenstone Terrane (Dall’Agnol et al. 1999). A detailed anisotropy of magnetic susceptibility
(AMS) study of one of these intrusions, the Redençao pluton, was carried out aiming to
understand the mechanism of emplacement of plutons in an intraplate setting not subject to
strong tectonic forces.
Field Relationships
The Jamon Suite is composed of 1.88 Ga granite batholiths and stocks (~5 to 50 km in
diameter) that were emplaced at shallow crustal levels (~1 to 3 kbar, Dall’Agnol et al. 2005).
All plutons are unfoliated and deformational structures are restricted to fracturing and
faulting. They cross-cut the E-W or NW-SE earlier structural trend of the Archean country
rocks and have caused hornblende hornfels contact metamorphism. The contacts are very
sharp, and angular xenoliths of country rocks are commonly observed near the margin of the
massifs. 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).
In this composite dike, a rhyolite porphyry shows evidence of mingling with an associated
mafic dike, demonstrating that the mafic and felsic magmas were contemporaneous. 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 petrography, geochemistry and magnetic petrology of the Redenção Granite were
discussed by Oliveira (2001). This subcircular granitic pluton is formed essentially of
monzogranites disposed in near-concentric zones and cut by syenogranite dykes. 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 that comprises roughly
fifty percent of the intrusion. 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 anellar structures in the central and southern
areas of the pluton. Aplitic dykes have very limited distribution and coincide in orientation
with the main faulting system. 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. Negative Eu anomalies increased with differentiation.
Anisotropy of Magnetic Susceptibility Study
Sampling and Measurements
Anisotropy of magnetic susceptibility (AMS) is particularly well adapted for the study
of granitic rocks, like the Redenção Granite, that show weak mineral shape-preferred
orientations. In this work, specimens obtained from 127 stations were studied for their AMS
according to the procedure described in Bouchez (1997). Cores were collected at an average
spacing of 1-2 km within the pluton. At least three specimens were retrieved from each
station, yielding a total of 780 specimens. The specimens were analyzed using a Kappabridge
KLY3 (AGICO) susceptometer, whose resolution is better than 10-8 SI, at the Laboratório
Helmo Rand (Universidade Federal de Pernambuco).
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 bulk magnetic susceptibility (K) has a unimodal distribution and
varies between 1.05x10-3 and 54.72x10-3 SI with an average of 11.55x10-3 SI. The high values
of K indicate that it is ferromagnetic in origin, in accord with the high abundance of
magnetite. The magnetic susceptibility decreases from the facies with higher modal contents
of mafic minerals to the leucogranites, i.e. from the border to the center of the pluton,
consistent with the geochemical zoning. The shape of the AMS ellipsoids, characterized by
the parameter T = [2(lnK2-lnK3)/(lnK1-lnK3)]-1 (Jelinek, 1981), varies widely, with oblate
ellipsoid (T>0) being slightly more abundant than prolate ones. The anisotropy degree (P)
varies from 1.02 to 1.47, but tend to be lower than 1.2, evidencing a low deformation intensity
in the solid state, which is confirmed by petrographic observations. P does not show clear
correlation either with rock type, K or T.
Magnetic Fabric
In the Redenção Granite, the AMS fabric is controlled mostly by magnetite, which is
an early-crystallized 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). which, considering the textural
relationships of magnetite with other mineral grains, can be interpreted as a magmatic flow
fabric. As a consequence, the magnetic lineation records the magma flow and corresponds to
the dominant magma stretching direction.
The orientation of magnetic foliations and lineations are shown in Figure 1. Magnetic
foliations have generally moderate to steep dips and magnetic lineations plunge gently to
moderately. In the even-grained coarse monzogranite facies, magnetic foliation and lineation
tend to be parallel to the contact of the pluton with the country rocks and the other facies.
They show a general concentric arrangement, coincident with the facies distribution in the
batholith. The magnetic lineation in the later emplaced seriated and porphyritic biotite
monzogranites tend to be oblique to the their contact with the coarse-grained facies. In the
central leucogranites, the magnetic fabric is apparently near concentrically disposed. However
it could have been also controlled by the dominant NE and NW structural trends.
Inferences on the emplacement mechanism and conclusions
The observed magnetic fabrics indicate a fairly well-defined magnetic foliation
parallel to the contact of the pluton with the country rocks and a less well defined but also
dominantly concentric orientation of the linear fabric. The lack of a dominant direction of the
linear fabric suggests a reduced influence of regional stress during granite emplacement, and
petrographic observations do not provide any evidence for subsolidus deformation and
recrystallization. Therefore, granite emplacement was not related to a major orogeny and its
magnetic fabric was not controlled by syn-emplacement regional deformation. Instead, the
AMS fabric reflects internal processes associated with magma transport and emplacement.
The dominance of subvertical magnetic foliations and steeply plunging magnetic lineations
suggests feeding by concentric dikes followed by horizontal flow of magma batches.
Acknowledgments
M.A. Oliveira and J.C. Almeida by support in the field and help in sampling. This research
received support from CNPq (RD- 550739/2001-7, 476075/2003-3, 307469/2003-4) and
UFPA. This work is a contribution to PRONEX/CNPq (Proj. 103/98-Proc. 66.2103/1998-0).
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