ARTICLE
TERRÆ 3(2):65-74, 2008
H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
Petrography, geochemistry and chemical
electron microprobe U-Pb-Th dating of
pegmatitic granites in Borborema Province,
North-eastern Brazil: a possible source of
the rare element granitic pegmatites.
Hartmut Beurlen1, Dieter Rhede2, Marcelo Reis Rodrigues da Silva1, Rainer Thomas2
and Ignez do Pinho Guimarães1.
Abstract
Keywords
Peraluminous, late to post-orogenetic
Brasiliano-Pan-african pegmatitic granites in the Borborema Pegmatite Province (BPP) in the Seridó belt,
northeastern Brazil were frequently cited as a possible
source of Ta-Be-Li-Sn-bearing granitic pegmatites. A
U-Pb monazite, with an age of 528 ± 12 Ma from
the Carnaúba dos Dantas–W pegmatitic granite was
induced to discard this particular intrusion as a possible
source of mineralized pegmatites, which have U-Pb
columbite ages within 509 - 515 Ma. New exposures
of pegmatitic granites in dimension stone quarries, allow
recognizing four petrographic facies identical to those described as source granites in pegmatite fields in Canada,
ie. facies rich in perthitic - graphic megacrysts; medium
to fine equigranular leucogranite; sodic banded aplite;
and pegmatitic facies. REE patterns are similar to their
Canadian counterparts. Preliminary chemical U-Th-Pb
electron microprobe analyses of uraninite and xenotime
from the largest pegmatitic granite in the BPP yielded
an age of 520 ± 10 Ma, which overlaps both the ages
of mineralized pegmatites and U-Pb monazite from the
recent literature. Columbite group minerals and cassiterite
were found as accessory phases in the aplitic facies of some
of these granites. Therefore, this granite type should still
be seriously considered as a possible source for mineralized
pegmatites.
Chemical U-Pb-Th dating; pegmatitic granites; Borborema Pegmatitic Province.
Resumo
Granitos pegmatíticos brasilianos, peraluminosos, tardi- a pós-orogenéticos ao BrasilianoPan-africano, foram frequentemente citados como
possíveis fontes dos pegmatitos graníticos portadores de mineralizações de Ta-Be-Li-Sn da Província
Pegmatítica da Borborema (BPP) na Faixa Seridó,
no Nordeste do Brasil. Dados de datação U-Pb em
monazita no granito pegmatítico de Carnaúba dos
Dantas-W com 528 ± 12 Ma induziram os autores
a descartarem esta intrusão como fonte possível
de pegmatitos mineralizados, datados entre 509 e
515 Ma pelo método U-Pb em columbita. Novas
pedreiras para explotação de granitos pegmatíticos
como pedra ornamental permitem o reconhecimento de quatro fácies leucocráticas, com texturas
e composições distintas: 1) fácies porfirítica, rica
em megacristais de pertita com intercrescimento
gráfico; 2) aplito sódico fino bandado; 3) fácies
pegmatítica; 4) fácies equigranular fina a média.
Estas facies são praticamente idênticas às descri65
H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
TERRÆ 3(2):65-74, 2008
tas para os granitos considerados como fonte dos
pegmatitos mineralizados em elementos raros no
Canadá. Os minerais acessórios destes granitos
incluem ferrocolumbita, manganocolumbita, cassiterita e gahnita. Os padrões de elementos de terras raras nestes granitos também são similares aos
dos exemplos canadenses. Dados geocronológicos
preliminares obtidos a partir das razões químicas
Pb/U/Th obtidas via microssonda eletrônica em
xenotima e uraninita indicaram uma idade de 520
± 10 Ma, coerente com as idades citadas na literatura recente para os pegmatitos mineralizados e de
U-Pb em monazita dos granitos pegmatíticos. Os
dados sugerem pois, que os granitos pegmatíticos
devem ainda ser considerados como possível fonte
dos pegmatitos mineralizados da PPB.
Borborema Province in northeastern Brazil, were
cited by many authors as a late phase of Neoproterozoic to Eopaleozoic (Cambrian to Ordovincian) granitic activity of the Brasiliano (= Panafrican) orogenic cycle (Lima et al. 1980, Torres
and Andrade 1975, Enes and Santos 1975). A few
authors recognized this peraluminous pegmatitic
granite type as related to or as a probable source of
Ta-Be-Li- bearing pegmatites of the Borborema
Pegmatite Province (BPP) (Da Silva 1993, Da Silva
et al. 1995, Araújo et al. 2001; Baumgartner et al.
2001). Despite this potential importance, probably
because of the usually small size of the intrusions
of these pegmatitic granites, most of them are not
represented in geological maps and have never been
studied or described in detail. Their geochemical
characteristics were used by Da Silva (1993), Da
Silva et al. (1995) and Baumgartner et al. (2006) to
identify a peraluminous character. A radiometric
U-Pb monazite age of 528 ± 12 Ma was obtained
by Baumgartner et al. (2001, 2006) in the pegmatitic granite intrusion approximately1 km west of
Introduction
Pegmatitic granites intruded in the Seridó
Belt in the central North Tectonic Domain of the
Figure 1- The Borborema Pegmatitic Province on a simplified geological base adapted from Brazil (1998, 2002), and
the location of the pegmatitic granite intrusions discussed in the text.
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TERRÆ 3(2):65-74, 2008
H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
the town Carnauba dos Dantas (N#. 5 in Fig. 1).
Based on U/Pb ages in columbite group minerals
of the pegmatites (509 ± 2.9 and 515 ± 1.1 Ma),
Baumgartner et al. (2006) suggested that this pegmatitic granite intrusion could not be the source
of the mineralized pegmatites.
The use of some of these pegmatitic granite
intrusions as dimension stones in the past few years
enabled the recognition that most of these intrusions are formed by four distinct compositionaltextural facies, very similar to those described by
Černý et al. (2005), as being typical of the source
granite of the Cat Lake - Winnipeg River pegmatite
field, in southeast Manitoba, Canada. According to
Černý et al. (2005), granites with very similar petrographic facies are considered source rocks of many
other examples of rare element granitic pegmatite
fields throughout the world e.g. Ontario and Quebec in Canada, Western Australia, South Dakota
and Colorado in USA, Finland, Sweden etc.
The petrographic description of some of these
pegmatitic granite intrusions, their geochemical characteristics and geochronological data in a
large intrusion will be discussed in this paper as
contributing to the evaluation of this granite type
as a possible source of the Ta-Be-Li-mineralized
granitic pegmatites of the BPP.
pegmatites of the BPP. The remaining 10% of the
pegmatites are intruded in Neoproterozoic granites, Jucurutu gneisses or basement rocks.
The numerous granite types found in the
Seridó belt and in the BPP were grouped by Jardim
de Sá et al. (1981) into four main phases, G1 to
G4, according to their relationship with the four
main deformation phases (D1 to D4) in the area.
Granites of the G1 group are represented by orthogneisses (mostly augen-gneisses), restricted to
the Paleoproterozoic basement of the Caicó Group,
and are deformed by the first (D1), and all subsequent deformational events. G2 granites, including
3 subgroups, correspond to pre- or early-tectonic
orthogneisses intruded into metassedimentary
rocks of the Seridó Group. They are intensively
deformed by isoclinal folding (thrust-related,
“tangential”, D2 deformation) and later events.
G3 granites (including subgroups G3A to G3C)
are syn- to late- tectonic with regards to the third
deformation event in the area (D3, characterized
by transcurrent shearing and upright, normal folding with vertical to sub-vertical axial planes). G3
granites usually show distinctive pervasive NNE
foliation. G4 granites are late to post-tectonic with
respect to D3 deformation, and show only localized weak signs of deformation related to NNE
shear zones. G1 and G2 granites predate the main
deformation and metamorphic phase of the Seridó
Group and can therefore be excluded as possible
sources of the mineralized pegmatites.
G3 and G4 granites (Jardim de Sá et al. 1981)
occur as several independent randomly distributed
intrusions. Field relationships do not allow any
correlation to be inferred between the pegmatite
mineralization and the intrusion(s). Lack of a large
central granitic intrusion and lack of an apparent
consistent zonal distribution of different pegmatite
types and subtypes surrounding this intrusion in
the BPP led some authors (e.g. Ebert 1969, 1970)
to interpret them as the result of partial melting of
rocks of the supracrustal sequence, as an alternative
to the igneous origin implicitly accepted by most
authors (Pough 1944; Scorza 1944; Johnston 1945;
Rolff, 1946).
The ages so far obtained for G3 Neoproterozoic granites in the SB range between 550 Ma and
610 Ma (Jardim de Sá 1994; Legrand et al. 1991,
1999; Jardim de Sá et al. 1986; Brito Neves et al.
2000, 2003). Ages of mineralized pegmatites ob-
Geology
The BPP overlaps the eastern and southeastern part of the Seridó Belt (SB) in the middle of
the North Tectonic Domain (north of the Patos
Lineament) of the Borborema Tectonic Province.
The SB is composed of a Brasiliano (630 Ma; Van
Schmus et al., 2003) supracrustal sequence known
as Seridó Group, which overlays a Paleoproterozoic
basement dominated by orthogneisses and migmatites (Caicó Group) with some small Archean
nuclei. This basement dominates the east and west
of the SB. The Seridó Group is composed of basal
gneisses, marbles, calc-silicate and amphibolites
of the Jucurutu Formation, overlain by quartzites
and metaconglomerates of the Equador Formation
and the topmost sillimanite – cordierite – garnetbearing biotite schist of the Seridó Formation. The
biotite schist, quartzites and metaconglomerates
are the host rocks of respectively 80% and 10%
of the more than 750 known mineralized granitic
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tained in the decades of the 1960s and 1970s range
from 460 to 530 Ma by several methods (U/Pb in
uraninite, Rb/Sr in K-feldspar and micas (Dirac
and Ebert 1967; Almeida et al. 1968, Ebert 1970).
More recently, Araújo et al. (2005) obtained an
Ar/Ar biotite age of 525 ± 2 Ma in mineralized
pegmatites. U/Pb columbite ages range between
509 ± 2.9 and 515 ± 1.1 Ma for five mineralized
pegmatites (Baumgartner et al. 2006). Taking into
consideration the usually fast crystallization rates
of rare element granitic pegmatites (Webber et al.,
2005), the ages of G3 granites group in the SB and
the lack of direct field relationship with the pegmatites, there seems to be a strong case against G3
granites being a source of the mineralized pegmatites of the BPP.
Da Silva (1993) distinguished several granite
types in the Picuí-Pedra Lavrada sub-area of the
BPP, respectively named GR1, GR2, GR3 (including facies GR3A and facies GR3B), GR 4 and GR5.
This nomenclature has no correspondence with the
G1 to G4 geotectonic granite groups of Jardim de
Sá et al. (1991).
GR3 granites of Da Silva (1993) are pegmatitic granites composed of GR3A facies, a medium
grained leucogranite, and GR3B, a pegmatitic facies, rich in K-feldspar megacrysts. Granites GR1,
GR3 and GR4 range in composition from granite
to granodiorite while GR2 varies from granodiorite
to tonalite, quartz monzodiorite and quartz diorite. This study focuses mainly on the petrographic
description of GR3 pegmatitic granites and a geochemical comparison with the other granite types
in the area, as well as with the geochemistry of pegmatitic granites supposed to be the source of rare
element granitic pegmatites in Ontario, Canada.
More than a dozen such intrusions have already
been recognized, but only four of them have been
examined with some detail, because their recent
use as dimension stones provided good sampling
conditions. The selected intrusions are: the Picuí
Pegmatitic Granite (Picuí quarry, no. 1 in Fig. 1),
10 km W of the town of Picuí, PB, a large intrusion, 4 km wide by 10 km long, extending along
the NS direction, parallel to the F3 foliation of
the enclosing biotite schists; the Marcação quarry
(No. 3 in Fig. 1), a dyke intruded 6 km south of
the Currais Novos city, striking NNW, 0.2 km wide
and 0.8 km long; a small intrusion 6 km N of the
Pedra Lavrada town (No 2 in Fig. 1), 0.1 km wide
and 0.3 km long; and the Galo Branco quarry (No
4 in Fig. 1), 2 km northeast of the town Equador,
at least 0.4 km wide and 0.5 km long, intruded in
quartzites of the Equador Formation.
The most prominent features common to most
of these intrusions are an overall leucocratic to
hololeucocratic character and decimeter to meter
sized compositional and textural banding, commonly with more or less regular cyclic repetition
of the different facies (Figures 2, 3).
The main textural-compositional facies distinguished in this granite type are:
1) A medium to coarse-grained equigranular
leucocratic granite, with grain size usually between
3 to 20 mm, composed essentially of K-feldspar,
albite or oligoclase, quartz and subordinated muscovite and some biotite. The biotite is frequently
chloritized. The main accessory minerals seen by
the naked eyes are garnet and sometimes black
tourmaline and magnetite. This facies is similar to
the “fine grained leucogranite” of Černý et al. (2005).
In some bands the micas form dendritic arrangements of centimeter-sized plates, oriented normal
to the overall banding, a feature reminiscent of that
described by Abella (1995) and Abella et al. (2003) as
an “arborescent” facies. The mica plates commonly
form sandwiches of muscovite and biotite.
2) An inequigranular leucogranite facies composed of very large (up to 100 cm), randomly
oriented megacrysts of perthitic microcline in
a medium-grained matrix similar to facies 1. A
graphic intergrowth of the feldspar megacrysts with
quartz is also a prominent feature of this facies.
This facies is equivalent to the pegmatitic leucogranite
distinguished by Černý et al. (2005). In some bands
the megacrysts may be oriented perpendicular to
the banding direction. Some of them show signs
Petrography of pegmatitic granites
Pegmatitic granite intrusions are widespread
in the BPP. They form small bodies, with less
than 1 km2 of exposed area, occurring usually as
dykes several hectometers wide and less than 1 km
long. This is probably the reason why intrusions
of this granite type are not represented in most
of the geological maps of the SB. However, even
some larger intrusions, with up to 4 to 40 km2
of exposure area, are omitted in many geological
maps of the SB, with the exception of the maps
by Enes and Santos (1975) and Lima et al. (1980).
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H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
A
B
D
C
Figure 2 - a Bench (13 m wide) of the Galo Branco quarry showing the metric layering of different facies of the pegmatitic
granite: 1) fine to medium grained equigranular leucogranite; 3) inequigranular leucogranite with graphic K-feldsparquartz megacrysts; 4) pegmatite pockets; b outcrop showing a sequence of facies 1), 2 (banded sodic aplite), and
3) (in this case with up to 1.2m long oriented megacrysts); c large polished plate (2 by 4 m) with gradational
transitions between facies 1 to 3 and- a discordant pocket of facies 4; d detail of the insert in c.
of blastic growth, because they may include relics (lines of garnet grains) of the “layered aplite”
facies. The megacrysts are often observed to be
zoned owing to variations in the color from cream
to pink (usually K-feldspar) or to white (commonly
albite). The graphic intergrowth may be restricted
to some zones of the megacrysts or vary in grain
size. In this case, the apparent grain size of quartz
usually increases from the center to the border of
the feldspar crystal. It is almost always clear that the
graphic intergrowth is the result of simultaneous,
epitaxial - syntaxial growth of feldspar and quartz
and that the apparent grains of quartz are part of
large dendritic crystals. At the contacts of the megacrysts with the matrix it is possible to observe that
the quartz of the graphic intergrowth at the border
of the megacryst forms a continuum, with the same
optical orientation, with the quartz of the matrix
(or with the quartz monocrystal forming the core
in the case of facies 4) (Figure 4).
3) An apparently equigranular fine grained
leucogranite with commonly mm-sized compositional banding, reflecting changes in the modal proportions of sodic plagioclase, perthitic microcline,
quartz and sometimes garnet or black tourmaline. A
similar facies is called by Černý et al. (2005) “layered
sodic aplite”. In some thin layers, millimeter to submillimeter sized garnet grains or black tourmaline
may be concentrated to more than 10, or rarely 30
% by volume, cases in which the fine compositional
banding is most prominent, and suggests an aplitic
or saccharoidal banded texture (Figure 5). Similar
garnet-rich aplitic banded facies are also observed
in the border zone of the mineralized pegmatites in
the BPP (Beurlen et al. 2007) and are described by
Webber et al. (1997, 2007) as “line rocks”, the most
commonly used designation in recent literature.
4) A pegmatitic facies composed of large oriented white or orange to pink perthitic megacrysts
enveloping a central mass of quartz. This facies may
occur as concordant veins, in which the megacrysts
are oriented symmetrically with respect to the
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TERRÆ 3(2):65-74, 2008
A
C
B
Figure 3 - a Bench (6 m high, 8 m wide) of the Marcação pegmatitic granite quarry howing the decimeter to meter-sized
layering formed by the alternation of equigranular fine (to medium) grained leucogranite facies (1), layered sodic
aplite or line rock (2), pegmatitic leucogranite (3) and pegmatite facies (4); b detail of a; c “blastic” growth
of zoned graphic poikilitic K-feldspar megacrysts (more than 30 cm in size) indicated by inclusion of lines of fine
grained garnet (relics of “line rock”).
central quartz mass. This feature distinguishes this
facies from the previous facies where the oriented
megacrysts are always oriented in same growth
vector. In other cases the pegmatitic facies forms
pockets or discordant veins. In all cases, contacts
with other facies are diffuse. The megacrysts are
frequently zoned with perthitic cores, some zones
with graphic quartz, and albitic rims.
The most important accessory minerals are
garnet (almandine according to SEM-EDS data)
and tourmaline, which can occur, concentrated in
some layers, as essential components. Other accessory minerals observed are zircon, xenotime,
monazite, apatite, magnetite, rutile, and more
rarely, cerianite, thorite, uraninite, ferrocolumbite
and manganocolumbite. The accessory minerals
occur mainly in facies 1 and 2 or in the matrix
of facies 3. They are usually recognizable only
under the microscope. In some thin sections
over 50 composite crystals of coaxial intergrowth
of zircon and xenotime with occasional uraninite and thorite inclusions are present and were
found (Figure 6), ideal for chemical U-Pb dating
purposes.
The presence of ferrocolumbite and, more
rarely, manganocolumbite as accessory minerals was observed in three of the four studied
intrusions (mineral identification confirmed by
SEM-EDS). In some magnetite crystals of the
Picuí quarry exsolution lamellae of pyrophanite,
gahnite, ilmenite and cassiterite were more rarely
observed. In the megacrysts and the quartz core
of facies 4, rare-earth-bearing phosphates, zircon
and columbite are very rare.
The main differences between the various pegmatitic granite intrusions in the BPP are the highly
variable relative amount of the four facies. The
rock-colors also vary from white or light gray (Galo
Branco, and another intrusion 1 km west of Carnaúba dos Dantas) to beige (Marcação quarry, Pedra
Lavrada N) or light reddish brown (Picuí quarry).
The feldspar color in the BPP is usually attributed
to dominance of either albite (white to gray) or Kfeldspar (beige to pink), and is supposedly related to
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H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
B
A
C
D
Figure 4 - a View of a bench of the Picuí pegmatitic granite quarry with decimeter to meter sized layering of facies 1
to 4 as described in Figure 3; b detailed view of the different facies; c and d polished thick sections respectively
under parallel and crossed polarized light, showing the contact between a graphic quartz + K-feldspar megacryst
(2.5 cm high) to the left and a supposed quartz vein to the right, showing that the formerly supposed vein-quartz
is a syntaxial monocrystal overgrown (see the slightly undulous extinction color) on the dendritic quartz in the
graphic intergrowth. The interference colors of the quartz are of second order because the section is more than
150 µm thick.
Fe-contents and a degree of alteration. This “field
rule” however, does not hold in the present case,
because the megacrysts in all studied pegmatitic
granite bodies are mostly formed by K-feldspar
independent of their color. In thin sections it is observed that the beige to reddish colors are the result
of impregnation by goethite along grain borders and
fractures within all minerals including quartz, and
along cleavage planes of feldspar. In some of these
micro-fissures relics of very fine-grained sulfides
were observed, supporting the suspicion that the
color of the granites is the result of alteration of
Fe-sulfides and not of the iron content in feldspar.
Differences in the amount of accessory or minor
main mineral components (garnet, black tourmaline, biotite) are also observed but do not account
for the color variations. While biotite is the main
colored component in the Picuí quarry (associated
with small amounts of garnet and tourmaline),
garnet is the most important colored component in
the Marcação, Galo Branco and Pedra Lavrada N
quarries, with subordinated biotite and tourmaline.
Black tourmaline is very common in the Carnaúba
dos Dantas-W intrusion.
Geochemistry
The pegmatitic granites studied are peraluminous to slightly metaluminous in the diagram
of Loiselle and Wones (1979) (Figure 7a) and in
Shand’s index plot according to Maniar and Piccoli (1989) of figure 8b. They have SiO2 contents
ranging from 72 to 78 % by weight. In comparison,
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H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
A
TERRÆ 3(2):65-74, 2008
B
C
Figure 5 - a A view of the Pedra Lavrada N pegmatitic granite outcrop showing the layering of the “line rock” facies
(2), the equigranular facies (1) and the megacryst facies (3). b detail of [a], showing a gradual coarsening of the
grain size in the graphic quartz-feldspar megacryst. c polished thick section under cross-polarized light, showing
a 1-cm K-feldspar with poikilitic inclusions of lines of round idiomorphic garnet grains (less than 0.5mm across).
The width of the photo (and of the feldspar crystal) is 1.2 cm.
Figure 6 - BSEI of two composite crystals of coaxial zircon (medium gray) and xenotime (slightly lighter gray) with
small uraninite (white) and thorite (light gray in the center of the grain on the right side). Note that the oscillatory
compositional zoning of lighter and darker zones is continuous in both zircon and xenotime, indicating an origin of
early primary crystallization from granitic melt, excluding the possibility of inheritance. Ages obtained from both
the uraninite and xenotime are the same. Uranium contents in zircon are too high to allow dating on this mineral.
White scale bar is 50 micrometers long.
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H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
A
B
Figure 7 - a Ternary plot according Loiselle and Wones (1979) for distinction between peraluminous, metaluminous and
peralkaline granites; b Distinction between metaluminous and peraluminous granitic rocks according to Debon and
Lefort (1988). In both diagrams the pegmatitic granites (including the equigranular medium grained leucocratic
facies GR3A and the pegmatitic facies GR3B) distinguish from other granite types GR1, GR2, GR 4 and GR5 in
the BPP (Da Silva 1993) due to their higher peraluminosity and more leucocratic character as revealed by lower
contents in Fe-Mg-Ti.
A
B
Figure 8 - Chemical characterization of the granitic rocks of the BPP (redrawn from Da Silva 1993)a in the silica
– alkalis plot according to Miyashiro (1978) and b in the Shand index plot according to Maniar and Piccoli
(1989).
the other granite types of the area are richer in Ca,
Fe and Mg and less peraluminous, as shown in the
A-B plot by Debon and Lefort (1988) of Figure 7b.
In the diagram of Miyashiro (1978), GR-2 granites
and GR-3B pegmatitic granites plot in the alkaline
field while the GR-3A (pegmatitic granites) and
other granites plot predominantly in the subalkaline field (Figure 8). GR2 granites show lower SiO2
contents, from 68 to 74 % by weight.
The chondrite normalized (Nakamura 1974)
REE patterns of the fine grained leucogranite and
the pegmatitic leucogranite facies are very similar
to those observed by Černý et al. (2005) in samples
of the Greer Lake intrusion in southeast ManitobaCanada (Fig. 9a and 9b). The REE patterns of the
studied pegmatitic granites are characterized by
strong negative Eu anomalies, low La/Lu ratios
(between 1 and 3), low total REE values (between
6 to 18ppm for facies GR3B and 36 to 90 ppm for
facies GR3A) and a disturbed character because of
random negative or positive anomalies of variable
intensity for Ce, Nd and Yb. The disturbed pattern is due to high content in fluxing elements (e.g.
P) according to Černý et al. (2005) and/or by early
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TERRÆ 3(2):65-74, 2008
A
B
Figure 9 - a REE patterns of granitic rocks in the BPP. The pegmatitic granites GR3A (open triangles) and B (solid
triangles) clearly distinguish from other granites in the area (GR2, stars in the vertical lined area, and GR1, GR4
and GR5 – small open circles in the horizontal lined area). b For comparison, the pegmatitic granites from Canada,
considered to be the source of mineralized pegmatites, present REE patterns (open triangles = pegmatite facies;
open circles = pegmatitic leucogranite; solid circles = equigranular fine grained leucogranite, solid triangles =
banded sodic aplite) similar to the GR3 counterparts in the BPP.
fractionation of xenotime and monazite possibly
concentrated as cumulates in some layers of banded
aplite facies. This pattern of REE distinguishes the
pegmatitic granite clearly from other granite types
in the BPP, which have higher ∑ REE, higher La/Lu
values (around 10) and no Nd anomaly (Fig. 9a).
The geochemical similarity of the pegmatitc
granites of the BPP with the Greer Lake intrusion
is also observed in the diagrams of normative corundum (CM) versus SiO2 weight %, normative
Qz-Ab-Or and An-Ab-Or diagrams (Fig. 10a, b).
Higher K/Rb, K/Cs and Al/Ga ratios recorded in
A
B
Figure 10 - a Ternary plots of normative Qz-Ab-Or and An-Ab-Or and b normative corundum (Cm) versus quartz plot of
pegmatitic granites GR3 A & B, from the BPP compared with those from Ontario-Canada according to Černý et
al. (2005)(same symbols of Fig. 9).
74
TERRÆ 3(2):65-74, 2008
H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
(1996), using a Jeol Hyperprobe JXA-850F in the
Geoforschungszentrum Potsdam, Germany. Operating conditions were 8 kv, specimen current of
10 nA, 30 to 40 nm beam diameter and 100 to 300
seconds acquisition time for the main peaks of U,
Th and Pb. The preliminary results are listed on
table 1. The results are very consistent and indicate
an age of 520 ± 10 Ma for the Picuí Pegmatitic
Granite crystallization. Table 1 Chemical U-Pb-Th
dating results by Electron microprobe . 1) Urn =
Uraninite, 2) Xnt = xenotime, 3) total low because
REE were not analyzed.
the granites of the BPP compared to those observed
in the Greer Lake intrusion indicate a lower degree
of fractionation.
Isotope geochemistry (U/Pb and Rb/Sr) in
feldspars (Baumgartner et al. 2006), indicate a multiple Pb source, with the main part derived from
old continental crust (> 2 Ga), for the Carnaúba
dos Dantas-W pegmatitic granite intrusion. This
assumption is based on the high and very variable
radiogenic lead values in different samples. Variable lead isotope ratios are also registered by these
authors in feldspar from six mineralized pegmatites
and interpreted as the result of variable degrees of
assimilation of different host rocks. This interpretation agrees with the supposition by Beurlen et al.
(2001) that high N2 proportions in the carbonic
phase of fluid inclusions in pegmatite minerals
from the BPP are the result of volatile assimilation
from wall rocks.
Conclusions
The crystallization age of 520 ± 10 Ma obtained
for the Picuí Pegmatitic Granite intrusion, the
largest pegmatitic granite in the BPP, seems to be
compatible with the supposition that this particular
pegmatitic granite type could be the source of the
mineralized pegmatites of the BPP. The uncertainty
in the age of the pegmatitic granite presented by
Baumgartner et al. (2006) overlaps the pegmatite Ar/
Ar biotites ages obtained by Araújo et al. (2005). The
data reported here for the Picuí Pegmatitic Granite
overlap both the data by Araújo et al. (2005) and
those by Baumgartner et al. (2006) for pegmatites
of the BPP. The spread of the age determinations
for both, the pegmatite and pegmatitic granite,
is still too large to allow the definitive confirmation that this pegmatitic granite type is the source
of the mineralized pegmatites. The presence of
columbite-group minerals and cassiterite as accessory minerals in the pegmatitic granites of the BPP
and their petrography and geochemical similarities
Chemical Pb-U-Th dating
A first attempt to determine pegmatite and
granitic pegmatite crystallization ages using U-Pb
zircon separated grains failed. As can be seen in
polished thin sections, zircon occurs either as very
long prismatic fine grained single grains or, most
commonly, as aggregates of several fine grained
zircon or composite crystals of zircon and xenotime
and/or monazite. Probably because of this pattern
(aggregates, needle-like shape) and small grain size,
most zircon, xenotime or monazite grains were lost
during grinding and heavy mineral separation.
Chemical dating was done by Electron Microprobe analysis of uraninite, xenotime or thorite,
following the method described by Rhede et al.
Table 1
Analized point
HB460102-K9
HB460102-K13
HB460102-K13
HB460102-K13
HB460102-K20
HB460102-K27
HB460102-K27
HB460102-K27
HB460102-K28
Mineral
Urn1)
Urn
Urn
Urn
Xnt2 )
Urn
Urn
Urn
Urn
ZrO2
0.00
0.27
0.15
0.19
0.00
0.20
0.20
0.27
0.02
ThO2
5.38
4.35
3.42
3.73
0.99
5.21
5.79
5.28
5.79
UO2
86.58
81.20
80.66
80.80
2.15
87.92
88.29
84.98
86.45
Y2O3
2.95
7.17
7.09
6.98
43.34
3.36
3.24
3.39
3.29
75
PbO
6.55
6.07
5.97
6.13
0.18
6.47
6.41
6.29
6.34
SiO2
0.00
0.00
0.00
0.00
1.19
0.00
0.00
0.00
0.00
P2O5
0.03
0.03
0.02
0.03
28.76
0.06
0.01
0.10
0.04
Total
101.49
99.09
97.30
97.85
76.603)
103.22
103.94
100.31
101.93
Age Ma
527
522
519
530
526
514
506
516
512
H. Beurlen, D. Rhede, M.R.R. da Silva, R. Thomas, I. P. Guimarães
TERRÆ 3(2):65-74, 2008
with source granites of other pegmatite fields in
Canada, Scandinavia and Australia are additional
arguments in favor of the relation of the pegmatitic
granite with the mineralized pegmatites. However,
more refined and voluminous geochronological
work on both pegmatites and possible source granites is certainly required.
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Baumgartner R.; Moritz R.; Romer R.; Sallet R. 2001.
Mineralogy and U/Pb geochronology of beryl
and columbo-tantalite pegmatites in the Serido
pegmatite district, northeastern Brazil. In: Mineral deposits at the beginning of the 21st century.
Proceedings of the 6th biennial SGA meeting,
Krakow, Poland, 26-29 August 2001, Balkema
Rotterdam, p. 371-375.
Baumgartner R.; Moritz R.; Romer R.; Sallet R. 2006.
Mineralogy and U/Pb geochronology of beryl and
columbo-tantalite pegmatites in the Serido pegmatite district, northeastern Brazil. Can. Mineral.,
44: (in press).
Beurlen H.; Da Silva M.R.R.; Castro C. 2001. Fluid
inclusion microthermometry in Be-Ta-(Li-Sn)bearing pegmatites from the Borborema Province,
Northeast Brazil. Chem. Geol.,173: 107-123.
Beurlen H.; Da Silva M.R.R.; Thomas R.; Soares
D.R.; Olivier P. 2007. Nb-Ta-(Ti-Sn)-oxide mineral chemistry as tracers of rare-element granitic
pegmatite fractionation in the Borborema Province, Northeast Brazil. Mineralium Deposita, 42;
in press.
Beurlen H.; Barreto S.B.; Martin R.F.; Melgarejo
J.; Rhede D.; Da Silva M.R.R.; Souza Neto J.A.
2007. The Borborema Pegmatitic Province in
Northeas Brazil: the state of the art. In: 3rd Int.
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art, Porto 2007, Memória 8 Fac. Cienc. Porto,
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BRASIL 1998. Mapa Geológico do Estado do Rio
Grande do Norte. Brasil, DNPM-CPRM/
UFRN
BRASIL 2002. Mapa Geológico do Estado da Paraíba.
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Brito Neves B.B.; Santos E.J.; Van Schmuss W.R.
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evolution of South America, Special Publication
31st International Geological Congress, Rio de
Janeirop, Brazil p. 151-182.
Brito Neves B.B; Passarelli C.R; Basei M.A.S.; Santos E.J. 2003. U-Pb zircon ages of some classic
granites of the Borborema Province. Geol. USP
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Černý P.; Masau M.; Goad B.E.; Ferreira, K. 2005.
The Greer Lake leucogranite Manitoba and the
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Acknowledgements
This study was possible due to the financial
support of the Brazilian Research Council - CNPq,
through grants APQ 470199/01 and PQ 352181/923 and, by CAPES (grant AEX 0728/04-7). We are
also indebted to Prof W. Heinrich of the GeoForschungsZentrum Potsdam (GFZ) in Germany for
allowing us to use of the Microprobe facility, to O.
Appelt from GFZ for technical support during the
microprobe analyses, to Bernardino R. Figueiredo
of the Instituto de Geociências of the University of
Campinas, Brazil (IGE-UNICAMP) for authorization and Dailto Silva for the SEM analyses at the
IGE-UNICAMP. Two anonymous reviewers are
greatly acknowledged for critical reading and very
helpful and constructive discussion.
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