A maior rede de estudos do Brasil

21 pág.
The complex hydrothermal history of granitc rocks

Pré-visualização | Página 1 de 11

The Complex Hydrothermal History of Granitic
Rocks: Multiple Feldspar Replacement Reactions
under Subsolidus Conditions
Recurring subsolidus re-equilibration of granitic feldspars induced by
fluid infiltration events provides a record of fluid^rock interactions
that affect large volumes of the Earth’s continental crust.This has a
direct bearing on the interpretation of the present-day granitic rock
mineralogy and geochemistry. We examine Palaeoproterozoic grey
and red-stained granitoids from the Simpevarp and Laxemar areas
in SE Sweden, particularly focusing on consecutive feldspar replace-
ment reactions, to provide an in-depth understanding of subsolidus
re-equilibration of granitic rocks with hydrothermal fluids. The
apparently most unaltered grey granitoids contain highly porous oli-
goclase grains that enclose crystallographically continuous microcline
relicts.This texture suggests that the oligoclase is already secondary
and may be a replacement product of original microcline. Oligoclase
is progressively replaced by albite (�An9) along polysynthetic twin-
ning and intragranular fractures. The features of this replacement
are characteristic of a dissolution^reprecipitation mechanism. Fine-
grained mica (sericite) is closely associated with the albite porosity
within micron-sized pores observable with scanning electron micros-
copy as well as in nanopores imaged with transmission electron
microscopy. The reddening phenomenon in the vicinity of fractures
is contemporaneously related to the K-feldspathization of sericite,
which is restricted to the altered oligoclase. Submicron size hematite
precipitation within orthoclase pores at the replacement front results
in the red coloration. The complex associations between the fluid^
feldspar reactions indicate that the replacement reactions may be
due to sequential fluid infiltration events and that the granitoids
have undergone extensive subsolidus re-equilibration, changing the
original magmatic mineralogy. Therefore, the effects of large-scale
re-equilibrations of granitic rocks through hydrothermal convection
systems should be more closely considered.
KEY WORDS: dissolution^reprecipitation; feldspar replacement; fluid^
rock interaction; granites; hydrothermal alteration
The hydrothermal alteration of granitic rocks indicates
considerable interaction between external fluid circulation
systems and crustal rocks on an enormous scale, as shown
by oxygen isotope studies (Taylor, 1977; Taylor & Forester,
1979; Hoefs & Emmermann, 1983). Recent research into
granitic rocks attributes their mineralogical and geochemi-
cal characteristics to purely magmatic processes and typi-
cally invokes multiple parental magma sources from the
mantle and crust as well as magma mixing and crystal
fractionation (Clemens & Vielzeuf, 1987; Millar et al.,
2001; Yang et al., 2004; Kemp et al., 2007). However, these
studies neglected to consider the effect of pervasive, large-
scale fluid^rock interaction and the resulting subsolidus
re-equilibration of granitic rocks with hydrothermal
fluids, despite clear indications of the hydrothermal reset-
ting of oxygen isotopes.
Hydrothermal activity is likely to be a more common
phenomenon in the continental crust than is currently
documented, as it is a natural consequence of any thermal
perturbation in fluid-rich rocks (Fyfe et al., 1978; Yardley
& Shmulovich, 1995). Pervasive fluid circulation systems
can extend throughout the crust and may influence several
cubic kilometres of rock. Conditions of pervasive metaso-
matism involving large rock volumes are well known in
mid-oceanic ridge environments (e.g. Alt et al., 1986), and
*Corresponding author. Present address: Physics of Geological
Processes (PGP), University of Oslo, Sem Selands vei 24, N-0316 Oslo,
Norway. E-mail: oliver.pluemper@fys.uio.no
� The Author 2009. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
JOURNALOFPETROLOGY VOLUME 50 NUMBER 5 PAGES 967^987 2009 doi:10.1093/petrology/egp028
nloaded from
/petrology/article-abstract/50/5/967/1604127 by U
PA user on 11 June 2019
are also responsible for the generation of many of the most
important ore deposits, including examples such as the
Olympic Dam Mine in South Australia (Haynes et al.,
1995; Barton & Johnson, 1996). The presence of crustal
hydrothermal activity is usually not accounted for, unless
it is associated with the formation of new minerals that
are characteristic of fluid infiltration, or when succes-
sive stages of replacement can be identified. A variety of
studies have been performed that address subsolidus re-
equilibration of granitic rocks by deuteric fluids, focusing
mainly on the coarsening of cryptoperthite to patch
perthite. This coarsening provides a simple petrographic
marker for fluid^rock interaction at subsolvus tempera-
tures (Parsons, 1978; Parsons & Brown, 1984;Worden et al.,
1990; Walker et al., 1995; Lee & Parsons, 1997; Parsons &
Lee, 2000; Nakano et al., 2002, 2005). Remarkably, more
general investigations of subsolidus re-equilibration of
granitic rocks in geochemical studies, using a broader vari-
ety of petrographic markers for fluid^rock interaction,
have not been undertaken. The albitization of plagioclase
and alkali feldspars is a possible petrographic marker as it
is one of the most frequently observed metasomatic alumi-
nosilicate reactions in the upper crust of the Earth, as has
recently been reviewed by Perez & Boles (2005).
Albitization has received limited attention, even though
this process probably marks the circulation of fluids of var-
ious origins on a very large scale. More generally, the
replacement of one feldspar by another is commonly asso-
ciated with fluid^rock interactions and metasomatism, as
Harlov et al. (1998) and Putnis et al. (2007a) have demon-
strated the replacement of plagioclase by K-feldspar.
Furthermore, the association of plagioclase alteration and
sericitization is a form of hydrothermal alteration found
extensively in granitoids (Que & Allen, 1996, and refer-
ences therein).
The red clouding in granitic rocks has long been attribu-
ted to the presence of ferric iron oxides in alkali feldspar
minerals (Boone, 1969; Taylor, 1977; Smith & Brown, 1988;
Nakano et al., 2002). Putnis et al. (2007a) recently demon-
strated that this red staining is due to the precipitation of
hematite from a fluid within the feldspar pores. The gener-
ation of porosity has been proposed as a fundamental
feature of a dissolution^reprecipitation mechanism for re-
equilibrating minerals in the presence of a fluid phase
(Putnis, 2002; Putnis et al., 2005, 2007, 2007a; Putnis &
Putnis, 2007b). Turbidity, a characteristic of replaced feld-
spars, originates from porosity and is almost ubiquitously
developed to varying extents within plutonic rocks
(Montgomery & Brace, 1975; Parsons, 1978; Worden et al.,
1990). Thus, the red clouding of feldspars can be used
as another direct marker for large-scale, subsolidus, re-
equilibrating crustal fluid flow.
Here we present work on granitoids from the Simpevarp
and Laxemar areas, SE Sweden, providing new data on
the re-equilibration of granitic rocks with crustal hydro-
thermal systems. Our study particularly emphasizes feld-
spar replacement reactions around fractures and their
textural and chemical changes. Numerous earlier studies
in the area and at the nearby A« spo« Hard Rock
Laboratory have illustrated that reddening in the vicinity
of fractures is the most prominent style of alteration of the
original grey granitoid (Drake et al., 2008, and references
therein). Initially the main aim of this work was to gain
an in-depth understanding