High-grade localized metasomatic alteration of the granitic
gneiss surrounding a clinopyroxene-rich pegmatoid dyke:
S€ ondrum stenhuggeriet, Halmstad, SW Sweden
D. E. HARLOV,
1,2
A. VAN DEN KERKHOF
3
AND L. JOHANSSON
4
1
GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473, Potsdam, Germany
2
Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa
(dharlov@gfz-potsdam.de)
3
Geowissenschaftliches Zentrum der Universit€ at G€ ottingen, Goldschmidtstrasse 3, D-37077, G€ ottingen, Germany
4
Department of Geology, University of Lund, S€ olvegatan 12, SE-22362, Lund, Sweden
ABSTRACT The S€ ondrum stone quarry (Halmstad, SW Sweden) exposes a transition from migmatized granitic
gneissic country rock into a foliated clinopyroxene-free granitic gneiss, and then a central pegmatoid
dyke dominated by clinopyroxene megacrysts. This transition zone represents a fracture-controlled,
fluid-alteration zone that developed under conditions of 650–700 °C and 790 MPa. Mineral chemical
trends in F, Cl, Fe, Ti, Mn and Y are interpreted as documenting a fluid infiltration event associated
with the formation of the pegmatoid dyke. Fluid inclusions from the pegmatoid dyke are CO
2
domi-
nant, whereas in the surrounding country rock they are dominated by H
2
O-NaCl-CaCl
2
. Fluid inclu-
sions from the intermediate foliated clinopyroxene-free granitic gneiss are a mixture of the two types.
The pegmatoid dyke appears to have originated from a high Ca activity, Fe-Mg enriched, fluid-rich
granitic melt with a CO
2
component, which was emplaced along a tectonic fracture in a regionally
migmatized granitic gneiss in the lower crust. This was accompanied by limited partial melting of the
surrounding granitic gneiss. The Ca activity of the melt was high enough to allow for the formation of
clinopyroxene megacrysts as opposed to orthopyroxene. H
2
O-enriched fluids expelled from the crystal-
lizing pegmatoid dyke, which retained the majority of the CO
2
helping to stabilize the clinopyroxene.
The expelled fluids coarsened and chemically affected the surrounding country rock resulting in trends
in the mineral and fluid inclusion chemistry seen today in the foliated and regional granitic gneiss.
Key words: clinopyroxene; fluid inclusions; granite pegmatoid; granulite facies; S€ ondrum stone
quarry; Swedish Granulite Region.
INTRODUCTION
In granitoid and mafic rocks, the stability of clinopy-
roxene relative to other ferro-magnesium minerals is
generally a function of the Ca activity governed to
some degree by the H
2
O activity, pressure and tem-
perature of the mineral assemblage present in the
rock. In that respect, depending on the minerals pres-
ent and the local geochemistry, clinopyroxene has a
relatively wide stability field ranging from 300 °C
and <100 MPa to lithospheric mantle pressures and
temperatures (Bird et al., 1984; Rivalenti et al., 1996;
Blundy & Dalton, 2000).
General reactions governing the stability of clino-
pyroxene relative to common OH-bearing minerals
such as biotite and amphibole include:
K(Fe,Mg)
3
AlSi
3
O
10
(OH)
2
biotite
þ CaAl
2
Si
2
O
8
anorthitite
þ 3SiO
2
quartz
¼ 2 (Fe,Mg)SiO
3
orthopyroxene
þ Ca(Fe,Mg)Si
2
O
6
clinopyroxene
þ KAlSi
3
O
8
K-feldspar
þ Al
2
SiO
5
þ H
2
O
(1)
and
KCa
2
(Fe,Mg)
4
Al
3
Si
6
O
22
(OH)
2
hornblende
þ 4SiO
2
quartz
¼ Ca(Fe,Mg)Si
2
O
6
clinopyroxene
þ 3 (Fe,Mg)SiO
3
orthopyroxene
þ CaAl
2
Si
2
O
8
anorthite
þ KAlSi
3
O
8
K-feldspar
þH
2
O
(2)
© 2014 John Wiley & Sons Ltd 389
J. metamorphic Geol., 2014, 32, 389–416 doi:10.1111/jmg.12077