DIATEXITES, SHOSHONITIC PLUTONS AND LOW-K QUARTZDIORITES AT THE SOUTHERN PART OF THE TAMBORILSANTA QUITÉRIA COMPLEX, NE BRAZIL: INSIGHTS INTO
MAGMA SOURCES AND TECTONICS
Felipe Grandjean da Costa1, Carlos Eduardo Ganade de Araújo1, Edney Smith de
Moraes Palheta1, Antonio Maurilio Vasconcelos1, Joseneusa Brilhante Rodrigues2
1
Geological Survey of Brazil, Fortaleza-CE (Felipe.Costa@cprm.gov.br), 2Geological
Survey of Brazil, Brasília-DF.
INTRODUCTION AND GEOLOGICAL SETTING
The Tamboril Santa Quitéria Complex (TSQC) is one of the largest
Neoproterozoic plutonism in the north Borborema Province. Recent geological
mapping, geochemical and geocronological data obtained by the Geological Survey of
Brazil at the west Ceará Central Domain (CCD) revealed a southwest continuity of this
magmatism, which extends more than 300 km on its NE-SW trend. This complex is an
anatetic/igneous association characterized by a number of magmatic pulses emplaced
between 650-610 Ma, (e.g., Fetter et al., 2003; Castro, 2004; Teixeira et al., 2005;
Santos et al., 2007). The plutonic rocks display an ubiquitous syn- to late-magmatic
low-angle deformation, and was in part coeval with the regional metamorphism
(Arthaud et al., 2008; Amaral, 2010). In general, the compositional predominance is
represented by granitic/granodioritic pink granitoids and diatexites, with some local
participation of less evolved dioritic to tonalitic rocks, and tholeitic gabbros (e.g., Fetter
et al., 2003; Santos et al., 2007). In this short communication we present U-Pb zircon
ages and geochemical information for a diatexite/shoshonite/quartz-diorite association
recently mapped at the southern part of the TSQC.
RESULTS
During geological mapping program in the scale 1:100.000 (Crateús Sheet) that
partly covered the southern portion of TSQC, three main plutonic associations were
separated according to their field, petrographic and geochemical characteristics: (i)
diatexites, (ii) shoshonitic plutons and (iii) low-K quartz-diorites.
(i) Pink diatexites (anatetic granites) are the predominant rock type of the area (up to
90%), they range from granodioritic to granitic compositions, and biotite is the main
mafic mineral phase observed. In the outcrops, these rocks usually show mafic to
intermediate partly molten enclaves (schollens) that may represent country-rocks or
infracrustal xenoliths. The high silica contents (SiO2 ~70 to 75 wt.%) and the slightly
peraluminous characteristic suggests melting of crustal sources, with similar
composition of their dioritic/tonalitic xenoliths. Therefore we speculate that this large
volume of anatetic magma probably derived from partial melting of a lower/middle
crust with a broadly dioritic/tonalitic composition. This anatetic granitic magmatism has
been wildly dated in literature with an age interval of ~650-610 Ma (e.g., Fetter et al.,
2003; Castro, 2004; Teixeira, 2005; Santos et al., 2007).
(ii) The Shoshonitic association is represented by monzonites, quartz-monzonites and
syenites that emplaced coevally with the anatetic granites. The common feature in these
syn-anatetic intrusions is the occurrence of mafic magmatic enclaves ranging from
monzo-gabbro to monzo-diorite in composition. These mafic magmatic enclaves are
potassium-rich rocks that are classified as shoshonites and ultrapotassic at the
discrimination diagrams. The plutonic rocks (monzonites/quartz-monzonites/syenites)
show SiO2 variations between ~56 and 68 wt.%, hornblende is the main mafic mineral,
and they all have metaluminous characteristic. For the mafic magmatic enclaves, silica
contents ranges from ~47 to 55 wt.% and they also have a metaluminous signature. At
least for the most primitive terms (i.e. SiO2 ~ 47 wt.% and MgO ~ 6,0 wt.%) of these
mafic magmatic enclaves, the magma source may be related to a mantle origin, and
probably an enriched lithospheric mantle to account for their high potassium contents.
For a quartz-monzonite example from this shoshonitic association we obtained a U-Pb
zircon age of 634 +/- 10 Ma (FC-162, 315160mE; 9416336mN). For these granitoids
(monzonites/quartz-monzonites/syenites), it is quite suggestive they origin from the
mixing between anatetic granites (diatexites) and mantle derived K-rich mafic magmas.
(iii) The quartz-diorites are low-K calc-alkaline rocks and represent the less evolved
plutonic rocks in the area, with silica variation between 53 to 63 wt.% and MgO
contents around 3,0 and 5,0 wt.%. These rocks are dominantly composed by plagioclase
(~50%), hornblende (~30%) and clinopyroxene (~15%), with minor amounts of quartz,
biotite and titanite. They occur as small intrusions within the granite/diatexite domain,
and one of these quartz-dioritic plutons yielded a U-Pb zircon age of 618 +/- 23 Ma
(CR-011, 294594mE; 9437217mN). This age makes it slightly younger, but closely
related in time to the anatetic granites and hybrid shoshonitic magmatism. The
geochemistry of the quartz-diorites is unusual compared to the diatexites and
shoshonites. Its dominant mafic composition precludes partial melting of felsic metaigneous rocks and partial melting of mafic sources seems necessary in order to explain
their relatively low-SiO2 and high-MgO contents. In general, there are two main
possibilities for the origin of these low-K calc-alkaline quartz-diorites: (1) melting of
mafic lower crust, or (2) melting of less enriched or even depleted mantle.
TECTONIC DISCUSSION
The linear occurrence of this plutonism, the parallelism with the Transbrasiliano
Lineament, and the close relationship with high-grade metamorphic rocks has drawn
attention of many researchers for interpreting its tectonic setting. The evolution of this
plutonic activity has been interpreted as the record of a continental magmatic arc around
650-610 Ma (Fetter et al., 2003), and recently, some authors have proposed a syn- to
late-collisional setting to this magmatism (Costa et al., 2010; de Araújo et al., 2010).
Geochronological constraints on regional metamorphism (Castro, 2004; Arthaud, 2007;
Amaral, 2010) match with the same interval (~650 to 610 Ma) of the plutonism at the
TSQC and suggest that metamorphism and magmatism essentially result from the same
thermal anomalies. Mafic plutonism is rare in the TSQC but can probably account for
elevated temperatures close to the peak of metamorphism because some tholeitic
gabbros, diorites and quartz-diorites are broadly coeval to the high-grade metamorphism
(e.g., dioritic gneiss 637+/-6 Ma, Fetter et al., 2003; metagabbro 623+/-5 Ma, Santos et
al., 2007; metagabbro 628+/- 1 Ma, Teixeira, 2005). The large amount of anatetic
granites (diatexites) in the TSQC, coeval with minor intrusions of gabbros, diorites,
quartz-diorites, syenites and quartz-monzonites, may be the melting results of distinct
crustal and mantle sources during an special thermal circumstances in orogenesis. For
this geodynamical scenery, Costa et al. (2010) proposed the model of Slab Breakoff for
the magmatic evolution of the TSQC, arguing that the delamination of a subducted
oceanic crust during Neoproterozoic continental collision allows good explanation for
combined high-grade metamorphism with diverse crustal and mantle derived
magmatism.
REFERENCES
Amaral, W.S., 2010. Analise geoquímica, geocronológica e geotermobarométrica das rochas de alto grau
metamórfico, adjacentes ao arco magmático de Santa Quitéria, NW da Província Borborema.
PhD. Thesis, University of Campinas (UNICAMP), 234 p. (in Portuguese with attached English
article)
Arthaud, M.H. 2007. Evolução neoproterozóica do Grupo Ceará (Domínio Ceará Central, NE Brasil): da
sedimentação à colisão continental brasiliana. PhD. Thesis, University of Brasília (Unb), 170p.
(in Portuguese with attached English article)
Arthaud, M.H., Caby, R., Fuck, R.A., Dantas, E.L., Parente, C.V., 2008. Geology of the northern
Borborema Province, NE Brazil and its correlation with Nigeria, NW Africa. In: West
Gondwana: Pre-Cenozoic Correlations Across the South Atlantic Region, Pankhurst, R.J.,
Trouw, R.A.J., Brito Neves, B.B., de Wit, M.J., (eds). Geological Society of London, Special
Publications, 294, 49-67.
Castro, N.A. 2004. Evolução Geológica Proterozóica da Região entre Madalena e Taperuaba, Domínio
Tectônico Ceará Central (Província Borborema). PhD. Thesis, University of São Paulo (USP),
221 p. (in Portuguese with english abstract)
Costa, F.G., de Araújo, C.E.G., Vasconcelos, A.M., Palheta, E.S.M., Justo, A.P. 2010. O Complexo
Tamboril-Santa
Quitéria:
Evidências
de
Slab
Breakoff
durante
colisão
continental
neoproterozóica, Norte da Província Borborema. 45º Congresso Brasileiro de Geologia, BelémPA. (in Portuguese)
de Araújo, C.E.G., Costa, F.G., Palheta, E.S.M., Cavalcante, J.C., Vasconcelos, A.M., Moura, C.A.V.
2010. 207Pb/206Pb Zircon ages of pré- and syn-collisional granitoids from the Tamboril-Santa
Quitéria granitic-migmatitic Complex, Ceará Central Domain, Borborema Province (NE Brazil):
Geodynamic Implications. VII South American Symposium on Isotope Geology, Brasília-Brasil.
Fetter, A. H., Santos, T.J.S., Van Schumus, W.R.,Hackspacher, P.C., Brito Neves, B.B., Arthaud, M.H.,
Nogueira Neto, J.A., Wernick, E. 2003. Evidence for Neoproterozoic continental arc magmatism
in the Santa Quitéria Batholith of Ceará State, NW Borborema Province, NE Brazil: implications
for the assembly of west Gondwana. Gondwana Research, 6(2): 265-273.
Santos, T.J.S., Dantas, E.L., Fuck, R.A., de Araújo, C.E.G., Rosa, F.F. 2007. The geology and U-Pb and
Sm-Nd geochronology from the north portion of the Santa Quitéria batholith, NE Brazil. XI
Simpósio Nacional de Estudos Tectônicos, V International Symposium on Tectonics of SBG,
Natal-RN, p. 142-144.
Teixeira, M.L.A. 2005. Integração de dados aerogeofísicos, geológicos e isotópicos no limite norte do
Complexo Tamboril-Santa Quitéria, CE (Província Borborema). Master Dissertation, University
of Brasilia (Unb), 197 p. (in Portuguese with English abstract)