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titanite crystal chemistry and up isotopic data: a petrogenetic
GEONOMOS 16(1): 29 - 36, 2008
29
TITANITE CRYSTAL CHEMISTRY AND U-Pb ISOTOPIC DATA: A
PETROGENETIC INDICATOR FOR PRECAMBRIAN GRANITOID
PLUTONS OF THE EASTERN BRAZILIAN SHIELD
Danielle Piuzana1#, Cristiane Castañeda1*, Carlos Maurício Noce1, Antônio Carlos Pedrosa
Soares1 & Luiz Carlos Silva2
Abstract
This paper presents crystal chemistry and U-Pb isotopic data for titanite (or sphene, CaTiSiO5) from
Precambrian granitoid plutons of distinct tectonic settings within the Eastern Brazilian shield. Despite the
fact that most titanite ages are related to partial lead-loss, resetting or crystallization during metamorphic
events, chemistry data reflect the magmatic history of the plutons, specially the Ti4+/Al+Fe3+ ratio. It is
demonstrated that for plutons of different crystallization ages (Archean, Paleoproterozoic and Neoproterozoic)
the measured Ti4+/Al+Fe3+ ratios of titanite grains are characteristic of the source region from which the
magma was extracted, and the ratios remain unchanged during subsequent metamorphic events. Higher
Ti4+/Al+Fe3+ values (> 9) are found in predominantly mantle-derived granitoid plutons whilst crust-derived
granitoid rocks have lower ratios. Therefore, titanite crystal chemistry may be used to discriminate magma
sources of granitoid rocks regardless their age and metamorphic history.
Introduction
Titanite can be an important petrogenetic indicator
since it can incorporate by partial substitution a variety
of chemical elements, such as REE, depending on the
mineralogical and tectnometamorphic history of rocks.
In addition, the phase relations between titanite and
other Ti bearing phases (mostly ilmenite and rutile)
are a function of bulk composition, pressure and/or
temperature (Lambert & Holland 1974, Enami et al.
1993, Frost et al. 2000, Kazonovitz & Wolf 2002),
and coexisting assemblage (Tropper et al. 2002).
However, the incorporation mechanism in titanite of
geochemically relevant elements is poorlly known
(Tiepolo et al. 2002), and also the existing relations
between these substitutions and petrogenesis.
The chemical composition of titanite can be
represented by the general formula CaTi(SiO 4 )
(O,OH,F), but it may deviate from its ideal composition
through the substitution of Ti and O by Al and F (Frank
& Spear 1985, Tropper et al. 2002). The Al + OH
substitution produces the end-member vuagnatite CaAlSiO4(OH) – a typical low-temperature phase with
a different structure than titanite (Enami et al. 1993). On
the other hand, the Al + F substitution is isostructural
and is common at high metamorphic temperatures and
pressures (Frank & Spear 1985). The tittanite structure
is generally described in term of chains of cornersharing octahedra, running parallel to the [100] axis and
cross-linked by isolated tetrahedra. This configuration
produces large cavities, where Ca ions in [7]-foldcoordinated sites lie (Deer et al. 1982).
In this paper, we discuss the crystal chemistry
of titanite and how isomorphic substitutions may be
related to its petrogenetic and metamorphic history.
The study is based on isotopic and crystal chemistry
data of titanites from the São Francisco Craton and
Araçuaí Orogen, in southeastern Brazil (figure1). The
analysed grains come from tonalitic- to granitic plutons
of distinct tectonic settings and magmatic ages.
Geological Setting and Sample
Descriptions
The granitoid bodies are found in two tectonic
settings, the southern part of the São Francisco Craton
and the Neoproterozoic Araçuaí Orogen developed
during the Brasiliano-Pan African Orogeny. The
cratonic basement consists of an Archean nuclei
displaying a classical association of TTG gneiss
complexes, greenstone belts and felsic plutons (e.g.
Teixeira et al. 2000), that is partially surrounded by a
Paleoproterozoic plutonic arc (e.g. Noce et al. 2000).
The Araçuaí Orogen developed at the southeastern
margin of the São Francisco Craton. Both Archean and
Paleoproterozoic rock associations can be traced into
the orogenic domain making up strongly reworked
basement units. The orogen also comprises the
volcano-sedimentary record of basin/orogen evolution,
ophiolitic remnants and a calc-alkaline magmatic arc.
Geochronological data for pre-, syn- and late-collisional
granitoids indicate that the orogenic stage lasted from
625 Ma to 560 Ma. A period of magmatic quiescence
was followed by intrusion of postcollisional plutons at
535–500 Ma. (Pedrosa-Soares et al. 2001).
Titanite were extracted from eight samples of
granitoid plutons, with magmatic ages ranging from
Archean to Neoproterozoic.
1 - CPMTC/UFMG, Brasil.
# - Currently in FACESA, UFVJM, Brasil - e-mail: [email protected]
* - Postdoctoral Fellow of CAPES/PRODOC
2
CPRM, Brasília, Brasil
30
Figure 1: Distribution of Archean, Paleoproterozoic and Neoprotezoic studied plutons in the Brazilian Shield
(modified of Pedrosa-Soares et al. 2001 and Silva et al. 2002).
31
Archean Granitoids
Sample N-57:
In the well-preserved Archean terrain within the
cratonic domain it is possible to identify an important
magmatic and tectonic event that took place at ca. 2790
to 2700 Ma, related to greenstone belt evolution and
the intrusion of several tonalitic to granitic plutons
(Machado & Carneiro 1992, Machado et al. 1992, Noce
et al. 2005). Sample N-57 came from a small pluton
dated at 2712+5/-4 Ma (Noce et al. 1998), cutting across
the much older TTG gneiss. It is a high K calc-alkaline
and slightly peraluminous granite related to a late- to
post-orogenic stage.
Samples LC-43 and LC-45:
The Guanhães Complex is a basement unit of the
Araçuaí Orogen with a protracted geologic evolution
that goes from Archean to Late Paleoproterozoic,
strongly reworked during the Brasiliano Orogeny. Both
samples are trondhjemitic-banded gneisses displaying
migmatitic structures, and must represent the older
TTG gneiss units within the complex. In spite of their
similarity, these trondhjemite gneiss gave distinct
crystallization ages of 2711±11 Ma (sample LC-43) and
2867±10 Ma (sample LC-45; Silva et al. 2002).
Sample LC-18:
This is a migmatitic banded gneiss with tonalitic
composition, dated at 2777±22 Ma (Silva et al. 2002).
This sample belongs to another basement unit of the
Araçuaí Orogen largely composed of Paleoproterozoic
plutonic rocks (the Mantiqueira Complex), but
including older inliers that cannot be distinguished
in the field from the country rocks due to pervasive
Neoproterozoic reworking.
Paleoproterozoic Granites
Samples N-18 and N-22:
Several Paleoproterozoic granitoid and mafic bodies
crop out along the southern border of the São Francisco
Craton, making up a magmatic arc partially preserved
from the Neoproterozoic tectonism. The plutonic bodies
display a wide range of composition, from gabrodiorite, TTG (tonalite-trondhjemite-granodiorite), and
granite (Quéméneur & Noce 2000, Noce et al. 2000).
Both samples come from the Alto Maranhão pluton,
a tonalite gneissic body that is interpreted as mantlederived according to its chemical and isotopic signatures
(Noce et al. 1997). Its crystallization age is given by a
concordant zircon age at 2130±2 Ma, whereas a titanite
discordia line yields an upper intercept at 2124±2 Ma
(Noce et al. 1998). The difference between zircon and
titanite ages may be ascribed to the intrusion-cooling
rate, or to a metamorphic event contemporaneous to
the magmatism.
Sample LC-16:
The Mantiqueira complex consists predominantly
of banded biotite-amphibole gneiss. The thickness
of the the alternating layers of felsic and more mafic
composition varies from centimetric to metric, and
concordant amphibolite boudins, lenses and layers
are very common. The felsic layers are of granitic to
tonalitic composition, and this unit is interpreted as the
product of intense deformation of the Paleoproterozoic
magmatic arc in the Araçuaí Orogen domain. Sample
LC-16 represents a tonalite layer of a banded
Mantiqueira gneiss displaying milonitic texture, and
yielded a magmatic age of 2052±26 Ma (Silva et al.
2002).
Neoproterozoic granites
Sample SM-8:
The Brasilândia pluton is a pre-collisional intrusion
of the Araçuaí Orogen, and consists of diorite, tonalite,
and granodiorite. Dating through the Pb-Pb evaporation
method yielded a minimum crystallization age of
595±3 Ma (Noce et al. 2000). Nd and Sr isotopic data
suggests a mixed magma-source involving a depleted
lower crust and some mantle contribution (Martins et
al. 2003). Sample SM-8c is a homogeneous foliated
tonalite, fine- to medium-grained.
Sample LC-38:
This sample is granodioritic gneiss with a poorly
constrained intrusion age of 565 ± 31 Ma (Silva et al.
2002). It contains tectonic lenses of garnet-sillimanite
paragneisses and may represent a syncollisional
intrusion of the Araçuaí Orogen.
analytical techniques
Titanite U-Pb analyses were carried out for this study
at the Geochronology Laboratory of the University of
Brasilia, with the exception of samples N-18/N-22 and
N-57 that were analyzed previously and presented in
Noce et al. 1998. Titanite concentrates were extracted
from rock samples using conventional gravimetric and
magnetic (Frantz isodynamic separator) techniques.
Final separation was achieved by hand picking using
a binocular microscope. Titanite grains were dissolved
in concentrated HF and HNO3 (HF:HNO3 = 4:1), and
a 205Pb–235U spike was used. Chemical extraction
followed standard anion exchange technique using
Teflon micro columns. Isotopic analyses were carried
out on a Finnigan MAT-262 multi-collector mass
spectrometer, and procedure blanks for Pb, at the time
of analyses, were better than 15 pg. For data reduction
and age calculation, PBDAT (Ludwig 1993) and
ISOPLOT-Ex (Ludwig 2001) were used. Calculated
ages are shown in table 1.
For crystal chemistry, fourteen single crystals
were analyzed by electron microprobe on a JEOL
JXA-8900RL spectrometer under the following
32
*1 – lower-intercept age
*2 – range of 206 Pb/238 U ages, discordant and/or
concordant analysis
*3 – range of207 Pb/206 Pb ages, discordant analysis
*4 – upper intercept age *5 – mean 206 Pb/238 U age
(a) U-Pb SHRIMP isotopic data from Silva et al.
(2002); (b)207 Pb/206 Pb in zircon crystals from Noce et
al. (2000); (c) Isotopic data from
Noce (1995)
operational conditions: 15 kV acceleration potential
and 20 nA sample current. The crystals were analyzed
using the following standards: jadeite (Na), asbestes
(K), hortonolite (Si, Fe, Mg, Mn), anorthite (Ca, Al),
chromite (Cr), synthetic glass (REE). Counting times
were 20s for REE and 16s for all other major elements.
The chemical data for each crystal are averages of 3-7
analyses taken uniformly over each crystal. The lower
limit of detection was 0.01 wt%. Calculation of the
titanite unit formula is a very difficult task due to the
33
Figure 2 a, b - The relationship between Ti4+ apf and Al+Fe+3 apf and the definition
of two groups A and B, respectively.
lack of accurate estimates of H, F, REE and other minor
elements. Iron is reported as Fe2O3. In this work, the unit
formula reported in Table 2 has been obtained based
on five oxygen atoms. The OH apfu was calculated
following Enami et al (1993).
Results
Isotopic data
Titanite crystals are near euhedral (N-57, LC43
and LC45) to subanedral (N-18, N-22, LC38, LC18,
LC16, SM8), ranging in size from 50 to 200 µm, and
no significant zoning was observed with the exception
of grains from sample N-57. Crystal morphology,
intragrain homogeneity and isotopic data are good
evidences for titanite crystallization in equilibrium with
the surrounding biotite and hornblende.
The wide titanite age spectrum is a consequence of
distinct magmatic ages and metamorphic histories of
studied plutons (Table 1). Granitoid plutons from the
cratonic domain were not affected by the Neoproterozoic
Brasiliano Orogeny, but record a Paleoproterozoic event
dated at ca. 2120-2050 Ma (Noce et al. 1998). Titanite
grains from the Archean granite (N-57) suffered partial
lead-loss due to this event, while titanite ages for the
Paleoproterozoic pluton (N-18/N-22) are close to the
intrusion age.
The Araçuaí Orogen records a syncollisional
tectono-metamorphic event dated at 585-560 Ma, and a
late orogenic thermal episode related to the gravitational
collapse of the orogen, dated at 535-500 Ma (Noce et
al. 2004). Titanite ages of the granitoid plutons located
within the orogenic domain are all Neoproterozoic,
regardless their magmatic age. For most samples, the
combination of resetting and/or partial lead-loss during
two thermal episodes resulted in a complex titanite age
pattern.
Mineral chemistry
A remarkable difference in titanite composition is
disclosed by two distinct groups of granitoid plutons,
especially concerning Ti4+, Al3+, Fe3+ and F- contents.
Titanite crystals from subduction-related plutons with
an important mantelic contribution (and/or mafic lower
crust) yield higher concentrations of Ti and lower
concentrations of Al+Fe3+, while opposite values are
found in titanites from high fractioned crust-derived
plutons (Table 2). The first group (Group A) comprises
34
Table 2 – Average chemical composition (wt%) and unit formula of titanite
Figure 3 a, b - OH- versus Al +Fe+3 and F- versus Al+Fe+3 substitutions respectively
35
Figure 4: Ca ions versus REE contents to the studied samples
Archean TTG gneiss samples (LC45, LC43 and LC18),
the Paleoproterozoic tonalite plutons (N-18/N-22 and
LC16), and the pre-collisional Brasilândia tonalite
of Neoproterozoic age (SM8). The second group
(Group B) includes a high-K Archean granite (N-57)
and two Neoproterozoic granitoid bodies related to
the sincollisional stage of the Araçuaí Orogen (LC38
and MU56). The two groups of granitoids can be
distinguished according to the titanite Ti4+/(Al+Fe3+)
ratio values. Group A has Ti4+/(Al+Fe3+) ratio values >
9, whereas Group B has lower values (< 9; Fig 2a,b).
It may be concluded that Al+Fe 3+ and Ti 4+
substitutions involve the entrance of F - and OH ions (Fig 3a,b). Titanite crystals from Group A
plutons plot roughly close to the Al-OH end-member
CaAlSiO4(OH), while titanite crystals from Group B
have a more variable composition. There is a negative
correlation between Al+Fe3+ and Ti4+ contents, and a
positive correlation between F- and OH- ions, implying
the substitutions can be coupled: CaTiO → Ca(AlFe3+)
FTi and Ca(AlFe3+)OHTi. In this context, a greater fluid
content seems to be present in crust-derived plutons
(Group B) because of the positive correlation between
F and OH contents and Al + Fe3+ contents.
In Group B Neoproterozoic plutons (samples LC-38
and Mu-56a), a fraction of the iron must have oxidized
during the generation of large volumes of fluids, and as
a consequence CaTiO → Ca(AlFe3+)F substitution took
place at the time of crystallization. The same hypothesis
can be admitted concerning the Group B Archean pluton
(sample N-57), but in this case the CaTiO → Ca(AlFe3+)
OH substitution occurred preferentially. This significant
fluid activity disclosed by crust-derived granitoids can
be related to fractioned melting. According to Frank
& Spear (1985), Carswell et al. (1996) and Markl
& Piazolo (1999), the It is considered a petrogenetic
indicator, according to studies developed on distinct
tectonic domains .
According to the chemical data (Table 2) a negative
correlation between Al+Fe3+ and Mn contents may also
be inferred, since there is no other correlation between
Mn2+ and Ca2+ ions. Si ions seem to be sufficient to
nearly full the tetrahedron site. Ca contents are similar
and invariable in all samples, ranging from 3.7 to 4.0
apfu, and no substitution can be established. There is
only a slight negative correlation between REE contents
and Ca ions. These evidences suggest that the entrance
of Al+Fe3+ at octahedral site can favors the incorporation
of smaller cations and available fluid, what can be well
identified by the nature of the rocks.
CONCLUSIONS
Titanite crystals from granitoid plutons with
magmatic ages ranging from Archean to Neoproterozoic,
and intruded into distinct tectonic domains, underwent
a complex history of partial lead-loss to resetting during
successive thermal events. Nevertheless, their original
bulk composition has been preserved. Analytical
techniques applied in this study show that Ti and Ca
sites of titanite crystal are independent since the Ca-site
is affected by tecnometamorphic events as evidenced by
U-Pb isotopic data while Ti-octaedral site substitutions
are directly related with the bulk composition or nature
of the rocks.
The majority of titanite crystal chemical analyses
for samples LC-38 and N-57, which are the most
fractionated granites, yield Ti4+/Al+Fe3+ <9. On the
other hand, TTG and tonalite samples presents Ti4+/
Al+Fe3+>9. Therefore, this ratio can be used as a
petrogenetic pattern to distinguish mantle from crustal
sources and/ or eventually mixing of sources.
Acknowledghments
The authors are indebted to the Brazilian
research-founding agencies: Conselho Nacional
de Pesquisa (CNPq) (Danielle Piuzana – Grant
300121/02-4) and CAPES/PRODOC (Cristiane
Castañeda).
36
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