Gondwana Research 9 (2006) 326 – 336
www.elsevier.com/locate/gr
207
Pb / 206Pb-ages of zircons from syn-collisional I-type porphyritic
granites of the central Ribeira belt, SE Brazil
Julio Cezar Mendes a,*,1, Ciro Alexandre Ávila b, Ronaldo Mello Pereira c,
Mônica P.L. Heilbron c,1, Candido A.V. Moura d,1
a
c
Departamento de Geologia, IGEO-Universidade Federal do Rio de Janeiro (UFRJ), Cidade Universitária, 21949-900, Rio de Janeiro, Brasil
b
Departamento de Geologia e Paleontologia, Museu Nacional, Universidade Federal do Rio de Janeiro (UFRJ),
Quinta da Boa Vista, 20940-040, Rio de Janeiro, Brasil
Faculdade de Geologia, Universidade do Estado do Rio de Janeiro (UERJ), Rua São Francisco Xavier, 524, 20550-013, Rio de Janeiro, Brasil
d
Departamento de Geoquı́mica e Petrologia, Universidade Federal do Pará, Rua Augusto Correa 1, 66075-110, Belém, Brasil
Received 15 February 2005; accepted 9 November 2005
Available online 10 January 2006
Abstract
In the central segment of the Ribeira belt, southeast Brazil, several foliated porphyritic granitic bodies intrude high-grade migmatitic gneisses
of the Andrelândia and Juiz de Fora domains and Embú Complex. Results of geological, geochemical and geochronological investigations of the
Maromba, Pedra Selada, Serra do Lagarto and Funil porphyritic I-type granites provide profound similarities, except for the distinct geochemical
behavior of the Funil Granite, perhaps related to a different crustal source. These granitoids show similar structural, textural and mineralogical
features. Pb-evaporation of single zircons provided ages of 586 T 6, 579.6 T 6.3, 586.3 T 4.8 and 584 T 5 Ma for the granites, respectively, coincident
with the syn-collision I episode of the central Ribeira belt. The intrusion of I-type porphyritic granitoids coeval with the main collisional event has
not often been reported in the geological literature. The most common syn-collisional granitic magmatism has normally an S-type signature or
even a slightly peraluminous I-type character. However, the occurrence of coeval I- and S-type syn-tectonic granites along the central Ribeira belt,
as observed in the investigated area and discussed in this paper is noteworthy.
D 2005 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Keywords: Central Ribeira belt; Neoproterozoic granites; Western Gondwana;
1. Introduction and tectonic setting
The Neoproterozoic Ribeira orogenic belt extends over
more than 1000 km along the southeastern coast of Brazil (Fig.
1). It developed as the result of the amalgamation of Western
Gondwana during the Brasiliano/PanAfrican orogenic episodes. Reported geochronological data indicate that orogenic
activity was diachronic along the supercontinent, ranging in
time from ca. 900 to 480 Ma.
Four major orogenic episodes have been recognized in the
Ribeira belt (Machado et al., 1996; Heilbron et al., 1995, 2000,
* Corresponding author.
E-mail addresses: julio@geologia.ufrj.br (J.C. Mendes),
avila@mn.ufrj.br (C.A. Ávila), rmello@uerj.br (R.M. Pereira),
heilbron@uerj.br (M.P.L. Heilbron), candido@ufpa.br (C.A.V. Moura).
1
CNPq (National Research Council).
207
Pb / 206Pb geochronology
2004; Trouw et al., 2000; Campos Neto, 2000; Heilbron and
Machado, 2003; Schmitt et al., 2004): a) 790 to 600 Ma
subduction and magmatic arc generation; b) 600 to 560 Ma
collision I episode; c) 530 to 510 Ma collision II episode; and
d) 510 to 480 Ma orogenic collapse.
Collision I episode (600 to 560 Ma) is the most important
orogenic event along the Ribeira belt and its northern
counterpart, the Araçuaı́ belt. This tectonic episode is
characterized by widespread generation of granitoids. Three
common types of granitoids, related to this episode, are
leucogranites, porphyritic (hornblende)-biotite granites and
hornblende granites.
Between the towns of Itatiaia, Visconde de Mauá and Santa
Rita de Jacutinga, in the Minas Gerais/Rio de Janeiro/São
Paulo state boundaries, voluminous and important porphyritic
granitoid magmatism was recognised and studied by Heilbron
(1993), Junho et al. (1999) and Pereira et al. (2001a). On a
1342-937X/$ - see front matter D 2005 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.gr.2005.11.004
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
327
Fig. 1. Tectonic map of Brazil modified after Schobbenhaus et al. (1984). Legend: 1—Phanerozoic basins; 2—Bambuı́ group; 3—Neoproterozoic orogens; 4—
Cratons (A—Amazon, B—São Luis, C—São Francisco, D—Luis Alves, E—Rio de la Plata).
regional scale, it is important to stress the occurrence of coeval
syn-collisional, I-type porphyritic and S-type granites in the
Ribeira belt.
The purpose of this paper is the presentation of new zircon
Pb-evaporation ages of four major plutons representative of the
porphyritic I-type syn-collision I magmatism of the belt
(Maromba, Pedra Selada, Serra do Lagarto and Funil) and
the discussion of their geological and geochemical data in the
context of the Gondwana amalgamation.
2. Tectonic organization and regional geology
The central segment of the Ribeira belt is subdivided in five
distinct tectonic terranes, in the sense of Howell (1989). These
are: Occidental, Paraı́ba do Sul, Embú, Oriental and Cabo Frio
Terranes (Heilbron et al., 2000, 2004). The investigated plutons
are located in the Occidental and Embú Terranes (Fig. 2).
The Occidental Terrane is subdivided in two structural
domains named Andrelândia and Juiz de Fora (Fig. 2). Both
contain three units: a) an older Paleoproterozoic basement (the
Mantiqueira Complex in the Andrelândia Domain and the Juiz
the Fora Complex in the Juiz de Fora Domain); b) a highly
deformed and metamorphic cover association (Andrelândia
megasequence) and c) Neoproterozoic granitoids. The Paleoproterozoic basement association includes hornblende and
biotite migmatitic orthogneisses (Mantiqueira complex) and
varied orthogranulites (Juiz de Fora complex). The Andrelândia megasequence includes banded biotite gneisses with
quartzites, amphibolites and pelitic gneisses, and sillimanite–
garnet – bitotite gneisses with several layers of calcsilicate
rocks, gondites, amphibolites and quartzites. As pointed out
by Junho et al. (1999), in the studied area, the extensive
migmatization transformed the paragneisses in stromatic
metatexites, crosscut by multiple leucosomatic veins, which
progressively grade into (garnet) – biotite – muscovite granitic
leucogneiss. In the northwestern part of the investigated area,
in the Carvalhos Klippe, high-pressure granulites were
described from the Andrelândia megasequence (Trouw et al.,
2000; Campos Neto and Caby, 2000).
The Paraı́ba do Sul and Embú Terranes also contain
basement and supracrustal associations (Eirado et al., in press).
In the former terrane, basement is represented by the Quirino
Complex and other orthogneisses, while the Neoproterozoic
cover, named Paraı́ba do Sul Group or Complex is composed
of pelitic gneisses, schists, calcsilicate rocks and marbles. In
the Embú Terrane, cover consists of tourmaline rich pelitic
gneisses and schists with quartzitic layers, while basement
associations are represented by hornblende orthogneisses
(Fernandes et al., 1990; Almeida et al., 1993; Pereira, 2001;
Heilbron et al., 2004).
Local structures are subdivided in two deformational pulses,
correlated to regional deformations D1 + D2 and D3 of Ribeiro
et al. (1995) and Heilbron et al. (1995, 2000). The oldest one,
D1 + D2, created the main foliation, locally mylonitic, and tight
to isoclinal inclined to subhortizontal folds that repeat and
thicken the units. In the Juiz de Fora domain, an important
328
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
Fig. 2. Simplified tectonic map of Santa Rita do Jacutinga – Resende – Bananal region, modified after Heilbron et al. (2004). Geological data were compiled from
Silva et al. (1992), Heilbron (1993), Ribeiro et al. (1995), Almeida et al. (1993), Pereira (2001), Eirado et al. (in press). Legend: 1—Alkaline Phanerozoic plutons;
2—Phanerozoic basins; 3—Syn-collision II granitoids (ca. 530 Ma); 4—S-type syn-collision I granitoids (ca. 580 Ma); 5—I-type syn-collision I granitoids (ca. 580
Ma); 6—Hornblende granitoids of uncertain age; 7—Embú Terrane; 8—Paraı́ba do Sul Terrane; 9 – 11 Structural domains of the Occidental Terrane: 9—Juiz de Fora
(JFD), 10—Andrelândia (ANDD) and 11—Carvalhos klippe of the Occidental Terrane; 12—Faults; 13—Normal faults; 14—Lateral shear zones; 15—Main
foliation; 16—Major thrusts; 17—Synform; 18—Antiform; 19—Geochronological sampling, with abbreviation of the name of the investigated plutons represented
by capital letters: M=Maromba; PS=Pedra Selada; SL=Serra do Lagarto; FU=Funil. Major towns are Bom Jardim de Minas (BM), Resende (RE), Volta Redonda
(VR), and Santa Rita de Jacutinga (SRJ).
tectonic imbrication mixed cover and basement rocks in several
thrust slices. The D3 phase refolded the previous structures,
generating open to tight folds with steep axial surfaces and SW
plunging axes. In the Andrelândia domain, in the northern
sector of the investigated area (Fig. 2), the interference between
D1 + D2 and D3 phases resulted in dome-and-basin and
refolded-fold patterns shown on the map. In the central and
southern sector of the area, D3 developed important subvertical
shear zones, such as the Além Paraı́ba, Alto da Fartura and
Cubatão shear zones.
The rocks of the region underwent syn-D2 barrovian
metamorphism, with increasing temperatures and medium
pressure (Heilbron, 1993, Trouw et al., 2000). The regional
metamorphism culminated with the formation of migmatites
and granites in the basal feldspathic metasedimentary rocks of
the Andrelândia megasequence (Ribeiro et al., 1995). As
mentioned above, the migmatitic rocks were subdivided into
two units grading into each other: a stromatic biotite gneiss
with metatexitic structures and a (garnet) – muscovite –biotite
diatexitic leucogneiss. Both are intruded by post-D2 finegrained equigranular leucogranite containing garnet, tourmaline, biotite and muscovite. It crops out as minor concordant
lenses or as dykes and sills. Both the diatexite and the intrusive
leucogranite result from progressive partial melting of the
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
metatexite and show typical peraluminous S-type characteristics (Junho, 1995), in contrast with the I-type foliated granites
studied in this work and described below.
3. Porphyritic granites
The Maromba, Pedra Selada, and Serra do Lagarto
porphyritic I-type granites occur, respectively, adjacent to the
Visconde de Mauá (Rio de Janeiro), Bocaina de Minas and
Santa Rita de Jacutinga (Minas Gerais) towns (Fig. 2) These
granitic bodies have a lenticular shape concordant with the
main southeast dipping regional S2 foliation (Heilbron et al.,
1995). The Serra do Lagarto granite is situated in the Juiz the
Fora domain, while the Maromba and Pedra Selada granites are
located in the Andrelândia domain. The Funil granite is located
in the Embú Terrane and extends from south of Areias to the
town of Itatiaia, where it disappears under sediments of the
Resende Basin (Fig. 2).
The predominant structure in the Maromba, Pedra Selada,
and Serra do Lagarto porphyritic I-type granites is a foliated
and lineated porphyritic fabric with a superimposed protomylonitic texture. The first structure is probably derived
from magmatic flow grading to solid state flow of a partially
crystallised magma (Heilbron, 1993; Junho et al., 1999). The
porphyritic texture is characterized by microcline megacrysts,
up to 3 cm long, within a medium grained granodioritic
matrix consisting of quartz, plagioclase, microcline, biotite,
hornblende, sphene, allanite, zircon and apatite. The abundance of polygonal quartz in the groundmass and in
fractures of microcline megacrysts indicates the more ductile
behavior of quartz in relation to feldspars. The Serra do
Lagarto granite shows the less deformed textures of the three
bodies.
The Maromba granite presents clearly intrusive contacts
with the surrounding biotite and muscovite bearing fine grained
equigranular S-type leucogranite (Almeida, 1996). It is limited
to the southwest by the Cretaceous alkaline Itatiaia massif and
to the northeast by steep fault contact with the diatexitic
leucogneiss. The contact with the metatexitic gneiss is
concordant.
The Pedra Selada granite is a large sill-like body, that
extends from south of Visconde de Mauá to the neighborhood
of Bocaina de Minas and Passa Vinte (Fig. 2). The contact with
the metatexitic paragneiss is sharp and concordant with the
regional foliation. Metric lenses of dark fine-grained quartzdiorite are found throughout but mainly along its borders.
South of Bocaina de Minas, this pluton contains small irregular
lenses of leucogranite gneiss with diffuse contacts.
East of the locality of Carlos Euler, the Serra do Lagarto
granite forms the Serra do Lagarto range (Heilbron, 1993). It is
also characterized by the presence of dark and fine-grained
quartzdioritic lenses, mainly along the borders, and displays
sharp and subconcordant contacts with the paragneisses and the
hornblende migmatitic gneiss of the surroundings. At the
eastern end several apophyses can be observed in the country
rocks, resulting in a lit par lit mesostructure (Junho et al.,
1999).
329
The Funil granite is a non-to weakly deformed body. It is
monzogranitic in composition and the porphyritic texture is
given by microcline megacrysts up to 7 cm long surrounded
by a fine to medium-grained groundmass composed of
quartz, zoned plagioclase, microcline and biotite. Apatite,
sphene, zircon, allanite and opaque minerals (magnetite,
pyrite, molibdenite and rare ilmenite) are accessory phases.
The orientation of microcline megacrysts and the aligned
quartz and biotite aggregates are considered to be markers of
the primary igneous foliation of the rock (Pereira et al.,
2001a).
Maromba and Funil granites show the lightest matrix colour
index and are devoid of dark quartzdioritic enclaves. A close
relationship with leucogranite and hornblende migmatitic
gneiss of the basement association, as observed in the Pedra
Selada and Serra do Lagarto granites, is missing.
Pereira et al. (2001b) obtained geochronological data for the
Mendanha and Funil porphyritic I-type granites, the former
situated northwest of the granites focused on this paper. They
achieved a 207Pb / 206Pb age of 592 T 5 Ma for the Mendanha
granite and 584 T 5 Ma for the Funil granite.
4. Geochemical data
Geochemical data of the porphyritic I-type granites were
reported by Heilbron (1993), Junho et al. (1999) and Pereira et
al. (2001a). Junho et al. (1999) recognised a clear geochemical
relation between the Maromba, Pedra Selada and Serra do
Lagarto I-type granites, characterized by high-K, calc-alkaline
and metaluminous to slightly peraluminous signature. They
interpreted the trends of the studied diagrams as a problabe
combination of crystal fractionation and/or magma mixing
process. The Funil granite was classified as an I-type as well,
indicating a slightly peraluminous, high-K calc-alkaline composition (Pereira et al., 2001a).
Fig. 3. AFM diagram for the porphyritic granites. Calc-alkaline boundary from
Irvine and Baragar (1971). Symbols: cross: Funil porphyritic I-type granite;
filled circle: Maromba porphyritic I-type granite; filled square: Pedra Selada
porphyritic I-type granite; filled triangle: Serra do Lagarto porphyritic I-type
granite. Source of geochemical data: Maromba, Pedra Selada and Serra do
Lagarto granites—Junho et al. (1999); Funil granite—Pereira et al. (2001a).
330
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
Fig. 4. Shand’s diagram for the porphyritic I-type granites. Symbols as in Fig.
3.
In this paper, geochemical data of porphyritic granites from
Heilbron (1993), Junho et al. (1999) and Pereira et al. (2001a)
are ploted in selected diagrams, as discussed below. The AFM
and Shand’s diagrams (Figs. 3 and 4) reinforce the calc-alkaline
and metaluminous to slightly peraluminous signature of the
granites. It must be emphasized that towards the most evolved
magmas of an acid I-type igneous sequence there is a tendency
to crystallize slightly peraluminous rocks (Clarke, 1981).
The Harker diagrams (Fig. 5) show the SiO2 range of the
granites. The more evolved character of the samples from
Maromba and Funil granites contrasts with the less evolved
from the Pedra Selada granite. From field and petrographic
observations, as well as linear trends observed in the
SiO2 Fe2O3, MgO, CaO, P2O5, Ba and Sr variation diagrams,
a genetic link of Maromba, Pedra Selada and Serra do Lagarto
granites may be inferred. The Funil granite represents the more
Fig. 5. Harker diagrams for the porphyritic I-type granites. Symbols as in Fig. 3.
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
331
Fig. 6. Chondrite-normalized REE diagrams for the porphyritic I-type granites. Symbols as in Fig. 3. Normalization values from Boynton (1984). (A) Funil Granite,
(B) Serra do Lagarto Granite, (C) Maromba Granite and (D) Pedra Selada Granite.
acid part of the trend, whereas the Pedra Selada granite forms
the more basic one. On the other hand, in some diagrams the
Funil granite samples tend to be clustered and do not fit very
well in the assumed trends (e.g. SiO2 Fe2O3, CaO, Ba and
Sr). This behavior is also apparent in Figs. 3, 6 and 7).
The preferential occurrence of quartzdioritic microgranular
enclaves in the Pedra Selada and Serra do Lagarto porphyritic
granites could indicate prior assimilation and/or contamination
process.
The REE patterns of Fig. 6 highlight the probable cogenetic
link between the Maromba, Pedra Selada and Serra do Lagarto
porphyritic I-type granites. They have quite similar REE
distribution pattern, showing to be highly fractionated with a
clear negative Eu anomaly. The Pedra Selada granite shows the
highest total REE content. REE concentrations of the Maromba
granite are lower than that of the Pedra Selada granite despite
its highest SiO2 content.
The well developed negative Eu anomaly in some patterns
of the Funil granite is possibly caused by feldspar extraction
during fractional crystallization processes. However, its less
fractionated REE patterns and the strong negative Eu anomaly
contrast with all others. The HREE enriched flat pattern may be
associated with sphene and zircon modal abundance, but
hybridization processes could also provoke changes in the REE
concentrations of the melt, as pointed out by Dini et al. (2004).
The different REE distribution of the Funil granite
reinforces its possible more evolved character, maybe related
to a different crustal source associated with the Embu Complex
host rocks, while the Maromba, Pedra Selada and Serra do
Lagarto granites are surrounded by the Andrelândia and Juiz de
Fora domains. Further achievement of isotopic Sr and Nd
analyses possibly will provide adequate elements to elucidate
this assumption.
In the R 1 R 2 diagram (Fig. 7), after Batchelor and Bowden
(1985), two trends can be envisaged: one from field 6 (syncollision field) to field 4 (late-orogenic field) and the other
from field 6 to field 7 (post-orogenic field). The former may be
associated with the formation of I-type granites during syn-to
late-orogenic processes. The alignment of granite samples
seems to reflect the evolution from less evolved metaluminous
to more evolved, slightly peraluminous granitic magma, in a
syn-tectonic environment.
5.
207
Pb / 206Pb geochronology
5.1. Sampling
Fig. 7. R 1 R 2 diagram for the porphyritic I-type granites. Symbols as in Fig. 3.
Fields from Batchelor and Bowden, 1985.
Zircon crystals were extracted from samples of Maromba,
Pedra Selada and Serra do Lagarto porphyritic I-type granites
(Fig. 2), after crushing, sieving, panning and concentration
using Frantz magnetic separation and heavy liquid (bromo-
332
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
Table 1
207
Pb / 206Pb data for single zircons of the Maromba Granite
Zircon
Description
Evaporation T (C-)
Measured ratios
204
207
207
Age (Ma)
1
euh, stpr, tr, py
2
euh, lgpr, tr, py
3‘
euh, lgpr, tr, py
4
5
6
euh, stpr, tr, py
euh, lgpr, tr, py
euh, stpr, tr, py
1450
1500
1550
1450
1500
1500
1550
1500
1450
1450#
1500
1500
1500
32
36
38
40
34
38
40
40
16
0
34
36
40
0.000050 T 39
0.000038 T 05
0.000108 T 20
0.000047 T 03
0.000057 T 11
0.000015 T 02
0.000038 T 04
0.000162 T 13
0.000202 T 06
0.000312 T 28
0.000104 T 09
0.000012 T 06
0.000031 T 07
0.06081 T 29
0.06029 T 22
0.06050 T 324
0.06033 T 08
0.06006 T 11
0.05993 T 24
0.06239 T 15
0.06174 T 28
0.06247 T 13
0.06198 T 25
0.06143 T 15
0.05990 T 13
0.06019 T 26
0.05984 T 41
0.05975 T 24
0.05901 T 21
0.05970 T 09
0.05925 T 10
0.05972 T 27
0.06183 T 09
0.05935 T 35
0.05957 T 21
0.05744 T 48
0.05958 T 54
0.05975 T 19
0.05963 T 35
598 T 15
595 T 09
568 T 08
593 T 03
576 T 04
594 T 10
669 T 03
580 T 13
588 T 08
509 T 18
589 T 20
595 T 07
591 T13
8
sbh, stpr, tr, py
14
euh, lgpr, tr, py
Average age
Pb / 206Pb
Pb / 206Pb
Pb / 206Pb*
Zircon age (Ma)
581.7 T 19.6
585.5 T 16.4
580 T 13
588 T 08
589 T 20
595 T 07
591 T13
586 T 6
Uncertainties are given at 2r.
#
Heating steps not used in the age calculation; *207Pb / 206Pb ratio corrected for common Pb contamination; ‘inherited zircon. Description of the zircon crystals:
euh=euhedral; sbh=subhedral; stpr=short prism (l / w between 1 and 3); lgpr=long prism (l / w > 3); tr=transparent; py=pale yellowish.
form). Zircons from the less magnetic fractions were selected
because they tend to be more concordant (Krogh, 1982).
The analyzed non-magnetic zircons are transparent to
occasionally translucent, colorless, pale yellowish or pink.
Bipyramidal prism length varies from long (width vs
length = 1 5) to short (width vs length = 1 2) and the grains
sometimes display subhedral morphology (Tables 1 2 and 3).
metamict zones of zircon are much less than from crystalline
domains (Kober, 1986). Thus, radiogenic Pb from metamict
zones can be purged at relatively lower evaporation temperatures, while radiogenic Pb from crystalline domains in zircon is
released only at higher temperatures (Ansdell and Kyser,
1993).
Isotope analyses on porphyritic I-type granites were
performed on a Finnigan MAT 262 thermo-ionization mass
spectrometer (TIMS) using the stepwise heating process
(Kober, 1986, 1987) with a double-filament arrangement
(evaporation and ionization). In this procedure, the zircon
was mounted on the canoe-shape rhenium evaporation filament, positioned in front of the rhenium ionization filament.
Initially, the ionization filament was heated, in order to be
cleaned. Afterward, the evaporation filament was heated
(evaporation step) to allow structurally bound radiogenic Pb
to diffuse out of the crystal. The Pb was deposited on the cold
ionization filament that was, subsequently, heated releasing the
deposited Pb for isotope determinations. The isotope data were
acquired dynamically using the ion counting system of the
instrument, with the intensity of the 207Pb beam between
30,000 and 100,000 counts per second (cps). The intensity of
different Pb isotopes were measured in the mass sequence
206
Pb, 207Pb, 208Pb, 206Pb, 207Pb, 204Pb. Ten (10) mass scan
5.2. Analytical techniques
Geochronological analyses by Pb-evaporation technique in
zircons (Kober, 1986, 1987) were carried out at the
Laboratory of Isotope Geology of the Federal University of
Pará (Pará-Iso).
Zircon is a heavy mineral with crystalline structure very
resistant to alteration and it tends to preserve the isotopic
information since its crystallization. Kober (1986) has shown
that the Pb components with the highest activation energy
normally reside in the intact crystalline zircon phase that shows
no post-crystallization Pb-loss and consequently yields concordant 207Pb / 206Pb ages. The progressive heating of a zircon
crystal during the Pb-evaporation techniques gradually releases
Pb isotopes. The theoretical basis for this technique is that the
activation energy necessary to release Pb from damaged or
Table 2
207
Pb / 206Pb data for single zircons of the Pedra Selada Granite
Zircon
Description
Evaporation T(C-)
Measured ratios
204
207
207
3
5
7
13
14
sbh, stpr, tl, py
euh, lgpr, tl, py
euh, stpr, tl, py
euh, lgpr, tl, py
euh, lgpr, tl, py
15
euh, lgpr, tl, py
1500
1500
1500
1500
1500
1550
1450
1500
34
28
30
30
32
8
32
16
0.000169 T 04
0.000106 T 16
0.000108 T 12
0.000048 T 05
0.000378 T 02
0.000395 T 28
0.000197 T 49
0.000040 T 04
0.06188 T 13
0.06077 T 28
0.06057 T 18
0.06005 T 19
0.06491 T18
0.06504 T 54
0.06217 T 36
0.06024 T 24
0.05946 T 18
0.05905 T 18
0.05886 T 37
0.05929 T 18
0.05948 T 25
0.05931 T 68
0.05965 T 42
0.05966 T 25
Average age
Pb / 206Pb
Pb / 206Pb
Pb / 206Pb
(*)
Age (Ma)
584 T 07
569 T 07
562 T 14
578 T 07
585 T 09
579 T 25
591 T15
591 T 09
Zircon age (Ma)
584 T 07
569 T 07
562 T 14
578 T 07
584.2 T 8.6
.3 T 7.7
579.6 T 6.3
Uncertainties are given at 2r.
*207Pb / 206Pb ratio corrected for common Pb contamination. Description of the zircon crystals: euh=euhedral; sbh=subhedral; stpr=short prism (l / w between 1 and
3); lgpr=long prism (l / w > 3); tl=translucent; py=pale yellowish.
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
333
Table 3
207
Pb / 206Pb data for single zircons of the Serra do Lagarto Granite
Zircon
Description
Evaporation T(C-)
Measured ratios
204
Pb / 206Pb
207
207
1
euh, lgpr, tr, cl
2
euh, stpr, tr, cl
3
euh, lgpr, tr, cl
4
euh, lgpr, tr, cl
5
euh, stpr, tr, cl
6
euh, lgpr, tr, cl
8‘
euh, stpr, tr, cl
9
euh, lgpr, tr, cl
10
euh, stpr, tr, cl
1450
1500
1450#
1500
1450
1500
1450
1500
1450#
1500
1450
1500
1450
1500
1550
1450
1500
1550
1450
1500
30
28
0
08
40
T38
36
32
0
36
16
38
34
28
34
36
38
14
30
30
0.000036 T 05
0.000038 T 04
0.000448 T 36
0.000136 T 08
0.000170 T 03
0.000150 T 09
0.000102 T 06
0.000156 T 06
0.001416 T 34
0.00131 T 04
0.000368 T 11
0.000114 T 05
0.000337 T 30
0.000030 T 07
0.000055 T 04
0.000084 T 06
0.000107 T 02
0.000118 T 21
0.000138 T 04
0.000027 T 02
0.05998 T 21
0.06029 T 17
0.06634 T 69
0.06176 T 35
0.06183 T 20
0.06130 T 13
0.06079 T 22
0.06122 T 27
0.07991 T 64
0.06174 T 18
0.06496 T 38
0.06155 T 18
0.09677 T 77
0.12076 T 14
0.12506 T 36
0.06062 T 17
0.06077 T 14
0.06084 T 31
0.06146 T 25
0.06017 T 21
0.05945 T 16
0.05948 T 22
0.05985 T 87
0.05979 T 37
0.05932 T 24
0.05962 T 29
0.05939 T 22
0.05920 T 16
0.05950 T 94
0.05980 T 19
0.05980 T 13
0.05981 T15
0.09209 T 88
0.12033 T 19
0.12429 T 37
0.05942 T 19
0.05924 T 12
0.05879 T 95
0.05949 T 25
0.05977 T 22
Average age
Pb / 206Pb
Pb / 206Pb
(*)
Age (Ma)
584 T 06
585 T 08
598 T 32
596 T 13
579 T 09
590 T 11
582 T 08
575 T 06
586 T 34
596 T 07
597 T 05
597 T 05
1470 T 18
1961 T 03
2019 T 05
583 T 07
576 T 04
560 T 35
589 T 09
596 T 08
Zircon age (Ma)
584.3 T 4.7
596.3 T 13.4
583.6 T 10.6
577.2 T 6.8
596.5 T 7.0
596.7 T 3.5
577.9 T 4.8
591.2 T 17.0
586.3 T 4.8
Uncertainties are given at 2r.
*207Pb / 206Pb ratio corrected for common Pb contamination; ‘inherited zircon. Description of the zircon crystals: euh=euhedral; stpr=short prism (l / w between 1 and
3); lgpr=long prism (l / w > 3); tr=transparent; cl=colorless.
define one block of data with eighteen (18) 207Pb / 206Pb ratios.
Discrepant isotopic values were eliminated using Dixon’s test.
The 207Pb / 206Pb ratios were measured in three steps of
evaporation at temperatures of 1450, 1500 and 1550 -C. The
average 207Pb / 206Pb ratio of each step was determined based
on five blocks of data. In general, the average 207Pb / 206Pb
ratios obtained in the highest evaporation temperature were
considered for age calculation of each zircon grain. However,
the ages of the lower temperature steps were also integrated
when they overlapped within error with those ages obtained at
the highest evaporation temperature. The uncertainties are
given in 2r errors. Common Pb corrections were made
according to the Stacey and Kramer (1975) two-stage model.
Only blocks with 204Pb / 206Pb ratios lower than 0.0004 were
used for apparent age determination of each evaporation step.
The data treatment followed the procedure described in
Gaudette et al. (1998).
In order to control the accuracy of the single zircon Pb
evaporation ages carried out in the Para-Iso, a number of
207
Pb / 206Pb apparent ages of the international standard zircon
95,100 were measured. The ages obtained varied from
1061.6 T 5.9 to 1077.2 T 5.9 Ma, giving an average age of
Fig. 8. Age (Ma) heating step diagram for zircon crystals of the Maromba porphyritic I-type granite. Filled circles: accepted blocks for age calculation; squares:
rejected blocks due to increasing or decreasing values of the 207Pb / 206Pb ratio. Uncertainties are given at 2r.
334
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
Fig. 9. Age (Ma) Heating step diagram for zircon crystals of the Pedra Selada porphyritic I-type granite. Filled circles: accepted blocks for age calculation; squares:
rejected blocks due to increasing or decreasing values of the 207Pb / 206Pb ratio; crosses: rejected blocks due to 204Pb / 206Pb > 0.0004. Uncertainties are given at 2r.
1069.8 T 2.3 Ma. The accepted age for this 95,100 zircon is
1065.4 T 0.3 Ma (Wiedenbeck et al., 1995).
5.3. New isotopic results
The zircon crystals of the Maromba porphyritic I-type
granite gave very consistent 207Pb / 206Pb apparent ages varying
between 589 T 20 and 580 T 13 Ma (Table 1 and Fig. 8). The
average 207Pb / 206Pb age, calculated based on 424 207Pb / 206Pb
ratios of seven zircons grains, is 586 T 6 Ma. In this body, one
crystal (zircon 3) yielded an age of 669 T 3 Ma that was
interpreted as an inherited crystal or a mixture of lead from an
old inherited core and a magmatic overgrowth.
The Pedra Selada porphyritic I-type granite yielded an
apparent 207Pb / 206Pb age of 579.6 T 6.3 Ma (Table 2 and Fig.
9). This age was calculated based on 210 207Pb / 206Pb ratios of
six zircons, whose ages vary from 562 T 14 to 591.3 T 7.7 Ma.
In the Serra do Lagarto porphyritic I-type granite, eight
zircon crystals gave 207Pb / 206Pb apparent ages varying
between 577.2 T 6.8 and 596.3 T 13.4 Ma (Table 3 and Fig.
10). The weighted-mean 207Pb / 206Pb apparent age of these
crystals, calculated based on 466 207Pbs / 206Pb ratios, is
586.3 T 4.8 Ma. One crystal (zircon 8) provided an age of
2019 T 5 Ma and is considered an inherited crystal or a mixture
of lead from an old inherited core and a magmatic overgrowth.
The lack of significant change in the 207Pb / 206Pb ratios on
progressive heating of analyzed grain 15 of the Pedra Selada
porphyritic I-type granite and grains 1, 2, 4, 6 and 10 of the
Serra do Lagarto porphyritic I-type granite suggests the
existence of only one stable radiogenic Pb phase.
Fig. 10. Age (Ma) heating step diagram for zircon crystals of the Serra do Lagarto porphyritic I-type granite. Filled circles: accepted blocks for age calculation;
squares: rejected blocks due to increasing or decreasing values of the 207Pb / 206Pb ratio; crosses: rejected blocks due to 204Pb / 206Pb > 0.0004. Uncertainties are
given at 2r.
J.C. Mendes et al. / Gondwana Research 9 (2006) 326 – 336
6. Discussion and conclusions
As suggested before by previous workers (Junho et al.,
1999; Heilbron et al., 1995, 2000; Trouw et al., 2000; Pereira et
al., 2001a), the geochemical as well as the new geochronological data confirm that the Maromba, Pedra Selada and Serra
do Lagarto porphyritic I-type granites are syn-tectonic and
crystallized and intruded during the D1 + D2 Ribeira main
deformation phase.
The common high-K calc-alkaline, metaluminous to slightly
peraluminous chemistry of the melt of Maromba, Pedra Selada
and Serra do Lagarto porphyritic granites is probably related to
a unique reservoir. Despite the lack of Sr and Nd isotopic data,
the granites geochemical behavior and minimum crystallization
age suggest such a relationship. The different REE pattern of
the Funil Granite is possibly associated with fusion of a
different protolith (perhaps related to the metasedimentary
rocks of the Embú Complex) and/or with the contribution of
accessory minerals (e.g. sphene, apatite and zircon) and
feldspar extraction during the fractional crystallization process.
The occurrence of I-type porphyritic granitic rocks yielding
ages contemporaneous with the main continental collisional
event has not often been reported in the geological literature.
The most common syn-collisional granitic magmatism has
normally an S-type signature, sometimes with a typical
aluminous paragenesis (Chappell and White, 1974), or even a
slightly peraluminous I-type character.
Janasi et al. (2001) discuss the relationship between granites
of the Agudos Grandes batholith, in the southern portion of the
Ribeira Belt in São Paulo State. They propose a scenario in
which I-type syn-collisional porphyritic granite (Ibiuna type,
with an age of 610 T 2 Ma) is closely associated with the
contemporary S-type Turvo granite (610 T 1 Ma). As described
before (Junho, 1995; Junho et al., 1999; Junho and Mendes,
2000), this situation is also observed in the central Ribeira belt,
where the I-type syn-collisional magmatism (Funil, Maromba,
Pedra Selada and Serra do Lagarto porphyritic granites) is
coeval with diatexites and associated leucogranites. This is also
demonstrated by coeval S-type granitoids of the Rio Turvo
batholith, cropping out in the investigated area, dated at about
580 Ma (Machado et al., 1996). Another example is the
Cantagalo leucogranite with an age of ca. 590 Ma (Heilbron
and Machado, 2003) that intrudes pre-collisional tonalitic
gneisses (about 630 Ma) of the Oriental Terrane of the central
Ribeira belt in Rio de Janeiro State.
Other examples of coeval S- and I-type syn-tectonic
porphyritic granites are reported from the Archaean basement
of southern India (Moyen et al., 2003) and from northern Brazil
(Barros et al., 2001). Toteu (1990) described a PanAfrican
metadioritic syn-collisional suite followed by late-collisional
porphyritic granite.
Also focusing on plutonic rocks formed during the
PanAfrican Orogeny, Van de Flierdt et al. (2003) investigated
the syn-orogenic quartz diorite –porphyritic granite/granodiorite –leucogranite association from the Bandombaai Complex,
Namibia. The porphyritic granite/granodiorite is slightly
peraluminous and contrasts with the strongly peraluminous
335
character of the leucogranite. Describing a geological setting
similar to the area focused in this paper, the authors emphasize
the production of these different magmas during the metamorphic peak.
A close relationship between syn-orogenic I-type granodiorites and tonalites and S-type granites is described by Pankhurst
et al. (2001) in the Famatinian orogenic belt from NW
Argentina, where the granitoids present an age of about 495
to 465 Ma.
The geochronological results of 584 T 5 Ma for the Funil
porphyritic granite and 592 T 5 Ma for the Mendanha porphyritic granite (Pereira et al., 2001b) are very close to the new
geochronological data presented here. This age interval,
synchronous with the central Ribeira belt deformational and
metamorphic peak, should be considered as an important
period of crustal magma generation, when the production of
granites showing contrasting mineralogy, textures and geochemical (I- and S-type signatures) characteristics was quite
significant.
In conclusion, the contemporary intrusion of S- and I-type
syn-collisional granitoids in the investigated area is probably
related to the structural style of the central Ribeira belt,
characterized by interfingering of cover and basement rocks on
a crustal scale (Heilbron and Machado, 2003, Trouw et al.,
2000, Schmitt et al., 2004, Heilbron et al., 2004). The
collisional crustal thickening may have induced widespread
melting of different sources, resulting in magmas with various
geochemical signatures.
Acknowledgements
The authors are grateful to GR referees Frank Söllner and
Randall Van Schmus for detailed reviews of the manuscript.
Special thanks to Maria C. B. Junho for her great support on
field trip activities. We are grateful to Dr. Claudio M. Valeriano
for critically reading the manuscript. Special acknowledgement
to Fundação Carlos Chagas Filho de Amparo à Pesquisa do Rio
de Janeiro (FAPERJ) for the financial support of the research.
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