G. Martins (1,*) , E. P. Oliveira (2) , C.R. de Souza Filho (3) &
J-M. Lafon (4).
(1) IG-DMG-UNICAMP & DG-UFC; ; (2) IG-DMG-UNICAMP & Isotope
Geochemistry Laboratory/ K.U. Lawrence, Kansas; (3)IG-DMG-UNICAMP; (4)
PARÁ-ISO/IG/UFPA.
The Algodoes Sequence crops
out between the towns of Quixada, Quixeramobim and Choro, Ceara State,
NE Brazil. It consists of a NNE-trending belt several kilometers-wide of
fine- to coarse grained mafic rocks (with minor ultramafic cumulates) intercalated
with metagreywackes, quartzites, metapelites and biotite-rich paragneisses.
Although field relationships are still unclear, a plutonic complex of tonalitic
to granodioritic composition is also associated with the Algodoes Sequence,
which we believe is intrusive into the latter. In close field relation
with the Algodoes Sequence there occur small plutons composed mostly of
partially migmatised granitic orthogneisses crosscut by dykes of
mafic to intermediate composition, and a kilometer-long granitic sheet
(Aroeira granite). The mafic rocks are dominantly represented by
massive amphibolites. No primary structures or textures have been
observed in the amphibolites so far that could help understand their tectonic
setting. However, occasional garnet-rich, garnet-poor compositional banding
strongly suggests that at least some of the amphibolites might have
originally been tuffaceous metavolcanics.
Structural features are
characterized by the presence of two transcurrent ductile shear zones with
dextral movement. These are characterized by flower or hemi-flower foliations,
stretching lineations with moderate to gentle plunge (00-300) and shear
and buckle-folds in meter to kilometer scale. These features indicate that
transpressive regime played a fundamental role to create the tectonic landscape,
to which the oblique collisional setting is suggested.
Preliminary age dating by
U-Pb and Pb evaporation on single-grain zircons yielded ages
of 2,123 Ma (Pb-Pb) and 2,131 Ma (U-Pb) for a tonalite, 2,172 Ma to 2,183
Ma (Pb) for granitic orthogneisses, and 2,054 Ma (U-Pb) for dykes of intermediate
composition. Because the dykes are mineralogically very similar to the
Aroeira granite, we assume that these rock units are coeval. TDM
Nd model ages for these rocks fall in the range 2,449 to 2,215 Ma. A four-point
whole-rock Nd isochron on amphibolites yielded and age of 2,236 ±55
Ma (MSWD=0.0897).
Major and trace element
geochemistry indicate a N-MORB to island-arc signature for the amphibolites,
and an arc-related, calc-alkaline trend for tonalites, granodiorites
and mafic to intermediate dykes. Epsilon Nd(t) values range from 2.13 to
2.85 for amphibolites (assuming an age of 2,2 Ga), 0.60 to 0.90 for
granitic orthogneisses and 1.53 to 1.89 for tonalitic orthogneisses. Fine
to coarse metagreywackes interbedded with amphibolitic horizons yields
epsilon Nd (2,2Ga) values ranging from 2,85 to 0,41. Although more
data must be acquired, altogether these new data strongly indicate additions
of juvenile, arc-related material to the crust of that portion of the Borborema
Province during the Paleoproterozoic.
(*) corresponde to guttenbe@ige.unicamp.br