Download pegmatites and pegmatites, just like granites and granites

PEGMATITES AND PEGMATITES, JUST LIKE GRANITES
AND GRANITES: APPLES AND ORANGES ARE JUST
NOT THE SAME!
Robert F. Martin1
Caterina de Vito2
Maria Sokolov3
1
Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada, bobm@
eps.mcgill.ca
2
Dipartimento di Scienze della Terra, Università di Roma 1 “La Sapienza”, P.le Aldo Moro 5,
I-00185 Roma, Italia
3
Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada
Keywords: orogenic suite, anorogenic suite, granitic pegmatite, source characteristics,
contamination, oxygen isotope geochemistry, pegmatites by anatexis, pegmatites by
fractional crystallization.
THE BIG
GRANITES
PICTURE
WITH
There is still a strong tendency out
there to lump! To many, the formation of
a granitic magma is strictly synonymous
with subduction of crust or overthickening
of crust owing to collision of continental
plates. The magmas are calc-alkaline in
character, and the plutonic and volcanic
rocks are metaluminous to peraluminous,
depending on the proportion of mantlederived and crust-derived components in
the source.
At the end of an orogeny, however,
there is commonly a tendency for the
overthickened crust to founder, and to
allow hot, asthenospheric mantle to rise
relatively quickly, such that a change
in tectonic setting can occur within
a short time! The diapiric rise of this
asthenospheric mantle is accompanied
by a metasomatic forerunner, a wave
of fluid affecting the uppermost mantle
(which was affected earlier in many cases,
one must recall, by subduction-related
Estudos Geológicos v. 19 (2), 2009
phenomena) and the crust, consisting
in part of young trench-fill sedimentary
units that have not been through an earlier
episode of prograde metamorphism. The
influx of hot fluid advecting in front of
the rising diapir causes anatexis in the
upper mantle and in the lower to middle
crust in a regime of extensional tectonics.
Thus one generates by anatexis a variety
of granitic and syenitic magmas of A type.
At the same time, the rising hot mantle is
subject to decompression melting, such
that a slightly alkaline basic magma can
form, and can differentiate to give a felsic
magma of A type. These two subtypes of
A-type granite are both distinct in many
ways from granitic magmas of orogenic
(calc-alkaline) associations (Martin 2006).
THE BIG PICTURE WITH LCT
PEGMATITES
Voluminous
batholiths
of
metaluminous to peraluminous calcalkaline granitic magma are generated at
crustal margins undergoing subduction
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PEGMATITES AND PEGMATITES, JUST LIKE GRANITES AND GRANITES: APPLES AND ORANGES
ARE JUST NOT THE SAME!
or collision. The ensuing pegmatites are
coded LCT because they are enriched in Li,
Cs and Ta, and in a host of other elements
that behave incompatibly in the presence
of the rock-forming minerals, and that
are progressively enriched in the residual
magma by crystallization of quartz and
the feldspars. Whereas some pegmatite
occurrences have a purely crustal
signature, others reveal a mixed crust–
mantle signature, just like their parental
batholiths. Pegmatites of LCT type that
have a strictly mantle signature are not
expected, nor are pegmatites in which the
a(CO2) was important. The characteristics
of the source of LCT pegmatites are
relatively well understood because these
are not very variable, and the petrogenetic
underpinnings of the suite are generally
well taught in university courses.
THE BIG PICTURE WITH NYF
GRANITIC PEGMATITES
The same cannot be said of NYF
granitic pegmatites, which are enriched in
Nb, Y and F, as well as in a host of other
high field-strength elements, and iron. These
elements were mobilized upward by the hot
H2O and CO2 coming in from the mantle; the
fluids fertilized the crust from below. Just as
A-type granites can have arisen by fractional
crystallization of basic magmas generated by
decompression melting of the rising diapir
OR by direct anatexis of the metasomatized
sialic basement, so can the NYF pegmatites
derived in this setting reveal a mantle or a
crustal isotopic signature. To understand
NYF pegmatites, it is thus essential to think
“outside the box” in terms of open-system
behavior PRIOR TO the formation of the
granitic melt. One quickly comes to realize
that in such a context, f(O2), a(SiO2), a(F),
a(CO2) are all important variables governing
the “signature” of the final product. For
example, as a carbothermal fluid is unable to
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transport SiO2, influx of a CO2-rich fluid is
likely to promote the formation of nepheline
syenite by anatexis, and silica-undersaturated
pegmatites or even carbonate-dominant
pegmatites may well be juxtaposed with
coeval standard NYF pegmatites elsewhere
along the same belt.
NYF PEGMATITES: CASES OF
CRUSTAL DERIVATION,
AND CASES OF MANTLE
DERIVATION
Our recent studies have focused on
the oxygen-isotope geochemistry of NYF
pegmatites. We have acquired δ18O values
on amazonitic K-feldspar and the coeval
albite in the subsolvus pocket assemblage
at the Anjanabonoina pegmatite, in
central Madagascar, earlier investigated
by De Vito et al. (2006). Values of δ18O
attain +15.3‰, which is unusually high
for a felsic igneous rock, and indicative
of a purely crustal source (Martin et al.
2008). Similarly for the Sakavalana NYF
pegmatite, located 100 km southwest of
Anjanabonoina, values on feldspar and
coexisting minerals are in the range 12.6
to 15.4‰. These data prove that NYF
pegmatites of anatectic origin do exist,
contrary to the predictions of some that all
NYF pegmatites are invariably related to
gabbro–granite sequences and fractional
crystallization imposed on a basic magma.
We have sought to test these
surprising results by acquiring such data on
other NYF pegmatites. We can report that
the amazonite and the pocket tourmaline
in the contaminated Minh Tien granitic
pegmatite (Sokolov & Martin 2009)
register above 15.8‰ and attain 16.4‰,
which is the world champion for a felsic
rock at present. Samples of amazonitic
microcline from the Paleozoic Morefield
pegmatite, in Virginia, studied by Sokolov
(2007), lie in the range +7.8–8.6‰, which
Estudos Geológicos v. 19 (2), 2009
Robert F. Martin et al.
may indicate a purely crustal source, but
more likely reflects a mixed source, i.e.,
possibly involving both crust and mantle
components. In contrast, there is the late
Grenvillian St-Ludger-de-Milot NYF
granitic pegmatite, near Roberval, Quebec,
in which δ18O is 3.2‰, quite consistent with
a modified mantle signature. Considered
in the same category are results for the
Proterozoic Keivy pegmatites, in the Kola
Peninsula, Russia, where the results are
systematically in the region 0 to –2‰!
These unexpected results may indicate
two- or three-stage anatexis of a mantlederived source rock.
We realize that there are amazing
amazonite-bearing granitic pegmatites in
Minas Gerais, for example at San Sebastião
de Pouso Alegre, investigated by Coutinho
(1947). A sample simply labeled Minas
Gerais, purchased from a dealer in North
America and characterized as “clean, green
amazonite” gave a value of 3.8‰, also
indicative of a modified mantle-derived
source. Whether or not it is typical of all NYF
pegmatites in Minas Gerais, and how these
values might differ from the LCT pegmatites
for which Minas Gerais is famous, are
interesting avenues for future research.
COMPLICATIONS
As Martin & De Vito (2005) have
shown, real life may be somewhat more
complicated than the above two-way
classification would suggest. There are
instances in the world where contamination
complicates immeasurably the classification
of a body of granitic pegmatite. For example,
a granitic pegmatite having an NYF
signature defined by its primary minerals
may undergo significant contamination
at the final stages of consolidation, when
thermal contraction or physical implosions
have caused a shattering of the wall zone.
Leakage of the caustic orthomagmatic fluid
Estudos Geológicos v. 19 (2), 2009
may well destabilize the rock-forming
minerals (plagioclase in particular) in the
wallrocks. Incursions of a fluid medium
from the wallrock may well lead to the
juxtaposition of elemental enrichments that
are rather unconventional. For example,
where the wallrock contains marble, as in
Anjanabonoina, Madagascar (De Vito et
al. 2006) and Luc Yen, northern Vietnam
(Sokolov & Martin 2009), Ca becomes an
important additive at the pocket stage. The
Anjanabonoina pegmatite is the type locality
of liddicoatite, a calcic tourmaline, but Ca
was a mere trace element in the evolved
NYF granitic magma. Our results so far
on the Ca-, Li- and B-bearing minerals that
postdate the NYF stage of crystallization do
not show δ18O results very different from
the early primary minerals.
TAKE-HOME MESSAGE
Mineralogical and geochemical
attributes of granitic pegmatites are the
result of an incredibly wide array of
physical and chemical variables. In this
review, an effort has been made to show
that all granitic pegmatites are not created
equal. The challenge is to address the
question of source characteristics of the
felsic magma, because these characteristics
are progressively amplified as the system
evolves, to be fully expressed at the end
stage of crystallization of the magma. It
is possible to make sense out of simple
LCT and simple NYF systems from the
point of view of source characteristics.
The likelihood that the systems become
open to country-rock contaminants late
in their history, however, because of
physical phenomena like implosions,
OR the aggressive chemical makeup of
the final batches of the orthomagmatic
fluid, OR both factors working in concert,
make contaminated granitic pegmatites
unusually challenging to understand fully.
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PEGMATITES AND PEGMATITES, JUST LIKE GRANITES AND GRANITES: APPLES AND ORANGES
ARE JUST NOT THE SAME!
REFERENCES
Coutinho, J.M.V. 1947. Amazonita
em Minas Gerais. Mineração e
Metalurgia XII, 68-70.
De Vito, C., Pezzotta, F., Ferrini,V. &
Aurisicchio, C. 2006. Nb–Ta–Ti
oxides in the gem-mineralized and
“hybrid” Anjanabonoina granitic
pegmatite, central Madagascar: a
record of magmatic and postmagmatic
events. Canadian Mineralogist 44,
87-103.
Martin, R.F. 2006. A-type granites
of crustal origin ultimately result
from open-system fenitizationtype reactions in an extensional
environment. Lithos 91, 125-136.
Martin, R.F. & De Vito, C. 2005. The
patterns of enrichment in felsic
14
pegmatites
ultimately
depend
on tectonic setting. Canadian
Mineralogist 43, 2027-2048.
Martin, R.F., De Vito, C. & Pezzotta, F.
2008. Why is amazonitic K-feldspar
an earmark of NYF-type granitic
pegmatites? Clues from hybrid
pegmatites in Madagascar. American
Mineralogist 93, 263-269.
Sokolov, M. 2007. Characterization
of Pb and selected trace elements
in amazonitic K-feldspar. M.Sc.
thesis, McGill University, Montreal,
Canada.
Sokolov, M. & Martin, R.F. 2009.
A Pb-dominant member of the
tourmaline group, Minh Tien granitic
pegmatite, Luc Yen district, Vietnam.
Fourth International Conference on
Pegmatite, Recife, Brazil.
Estudos Geológicos v. 19 (2), 2009