Fig. 3, 1 have a pronounced foliation made up by alternating platy quartz and bands containing
fine-grained, brown biotite. The quartz plates have internal CS-fabrics defined by optically ori-
ented, dynamically recrystallised quartz crystals, i.e. microstructures typical of dislocation creep.
Kinematic indicators are fairly common, includ- ing composite asymmetric augen, s-porphyro-
clasts and locally existing C at a low angle to the mylonitic foliation C, Fig. 6E. K-feldspar por-
phyroclasts
generally show
intense static?
sericitisation. An important condition is that the ultramylonites lack chlorite and epidote minerals,
i.e. exhibit no records of retrograde, low tempera- ture deformation.
In the metagreywackes ca. 200 m from the contact to the granitoid, a pseudoclastic texture is
still visible in spite of strong shearing. The defor- mational fabric contains dynamically recrys-
tallised quartz plates in which the crystals indicate grain boundary migration recrystallisation cf.
above. Fine-grained, brown biotite dominates over white mica, but larger muscovite fish with
internal strain undulose extinction and rare kink- bands exist Fig. 6F.
The microstructures in the adjacent metasedi- mentary rocks are, thus, comparable with those in
the mylonitised granitoid, suggesting that they formed simultaneously along a major shear zone.
The comparatively low metamorphic grade of the greywacke close to the intrusion, including a pseu-
doclastic texture, indicates that the greywacke has been juxtaposed by post-emplacement shearing.
4. U – Pb geochronology
4
.
1
. Analytical method A sample of the recrystallised CS-mylonite
from the Forsaa˚n shear zone Swedish national grid, 697661145656 was collected for U – Pb ti-
tanite thermal
ionisation mass
spectrometry TIMS and zircon secondary ion mass spec-
trometry SIMS analyses to investigate both the timing of shearing and the protolith age. As a
reference sample, the undeformed protolith was also collected for U – Pb titanite analysis. The
sampling site of the latter is ca. 1 km west of the Forsaa˚n zone Swedish national grid, 697210
145676 within one of the type areas for Revsund granitoids as defined by Ho¨gbom 1894 Fig. 3.
The majority of the titanites from the CS-my- lonite are smaller than 100 mm and oblate shaped.
Due to crushing and milling, many of the crystals were fragmented. Approximately 30 transparent
crystals and fragments, free from inclusions and cracks and with a total weight of 460 mg were
selected for U – Pb analyses. They were dissolved in HF:HNO
3
in a Savillex
®
beaker on a hot plate for ca. 70 h. The sample was evaporated and HCl
added before it was aliquoted. One aliquot was spiked with a
208
Pb –
233
U –
235
U tracer, and U and Pb were separated using a HBr and HNO
3
ion exchange technique.
The titanites from the undeformed granitoid are distinctly different from those of the mylonite.
They are generally larger 200 – 250 mm and euhe- dral with well-developed crystal faces with sharp
edges. Due to their large size, the crystals are almost opaque and to avoid problems with hidden
impurities and inhomogeneous parts, only clear and inclusion free, ca. 150 mm large fragments
ca. 15 corresponding to ca. 1 mg were selected for analysis. In this case, they were dissolved in an
autoclave in 205°C for 50 h and a
205
Pb –
233
U –
235
U spike were used as a tracer, while the rest of the procedure was as above.
The U – Pb analyses were carried out on a Finnigan MAT 261 solid source mass spectrome-
ter at the Swedish Museum of Natural History in Stockholm. Corrected isotope values, UPb, Pb
Pb ratios and intercept ages were calculated using the program by Ludwig 1993, 1995. The initial
lead correction was made according to Stacey and Kramers 1975 and the decay constants recom-
mended by Steiger and Ja¨ger 1977 were used.
The zircons from the Forsaa˚n CS-mylonite are pale pink in colour and generally have a short
prismatic habit. They have well-developed crystal faces with very sharp edges and high lustre. No
cores were detected with a binocular microscope, but cathodoluminescence images showed that
cores were present in some crystals, while others have regular zonation patterns typical for igneous
zircons
Vavra, 1990
or slightly
irregular patterns.
Fig. 7. A U – Pb concordia diagram from TIMS titanite analyses showing that the undeformed granitoid and the For-
saa˚n shear zone yield ages of 1852 9 2 and 1816 9 2 Ma, respectively. The titanites from the former are \ 250 mm and
euhedral with sharp crystal faces, whereas the latter are B 100
m m and oblate shaped. Microstructures indicate that the titan-
ites from the Forsaa˚n zone grew during deformation and the age is therefore interpreted to represent the ductile shearing.
Size of the error ellipsoid and uncertainties are given with 95 confidence. Isotopic data are presented in Table 1. B U – Pb
concordia diagram for SIMS zircon data from the Forsaa˚n zone. A regression through ten analyses in the central part of
the crystals yields an age of 1849 9 14 Ma. The four shaded ellipses show analyses at the margin of the zircons and have
been omitted from the regression. The relative probability plot inserted diagram shows a slight uneven distribution, indicat-
ing a younger overprintinglead loss. Sizes of the error ellip- soids are given in 1s and uncertainties with 95 confidence.
Isotopic data are presented in Table 1.
The zircons selected for U – Pb SIMS analyses were mounted in transparent epoxy resin together
with chips of reference zircon 91500 with an age of 1065 Ma Wiedenbeck et al., 1995. The sample was
polished to reveal as much of the mounted zircons as possible and coated with ca. 25 nm of gold. The
analyses were performed at the Swedish Museum of Natural History, Stockholm, using the NORD-
SIM Cameca IMS 1270 ion microprobe. A 4 nA O
2 −
primary beam producing an ellipsoid analysis spot size of approximately 25 mm was used, and a
single electron multiplier in an ion counting mode measuring following masses,
90
Zr
2 16
O 196,
204
Pb 204, background 204.2,
206
Pb 206,
207
Pb 207,
208
Pb 208,
238
U 238,
232
Th
16
O 248,
238
U
16
O
2
270. Detailed descriptions of the analytical and calibration procedures are given by Whitehouse et
al. 1997, 1999. Corrected isotope values, using modern common lead composition Stacey and
Kramers, 1975 and measured
204
Pb, UPb and PbPb ratios, and intercept ages were calculated
using the IsoplotEx ver. 2.00 Ludwig, 1999. The ages are calculated using the decay constants rec-
ommended by Steiger and Ja¨ger 1977. For the regression, different line fitting models are recom-
mended by Ludwig 1999. In this case, Model 1 has been used as the MSWD mean square of weighted
deviates value is relatively low. This means that the scatter is assigned to analytical errors and error
correlation only.
4
.
2
. Results The titanites from the CS-mylonite in the For-
saa˚n shear zone yield an almost concordant result of 1816 9 2 Ma Fig. 7A. The best estimate of the
age is given by the
207
Pb –
206
Pb data on which the diminutive discordance has an insignificant influ-
ence. The age is interpreted to be that of the deforma-
tion. There are two specific reasons for this inter- pretation,
which are
both based
on the
microstructures recorded. 1. The titanites are part of the deformational
fabric, i.e. they grewrecrystallised during the deformation, which resulted in a thorough re-
working of the granitoid in the deformation zones.
2. The microstructures indicate that deformation took place below the 600 – 700°C closure tem-
perature for Pb diffusion in titanite of this size Scott and St-Onge, 1995, which means that
no substantial diffusional Pb-loss occurred af- ter the titanite formation.
The titanites from the undeformed reference sample of the granitoid yield a considerably
higher age than the titanites from the mylonite. The U – Pb analysis is slightly discordant with a
207
Pb
206
Pb age of 1851 9 2 Ma Fig. 7A. Cathodoluminescence images on zircon from
the Forsaa˚n CS-mylonite show a large variation of internal structures including inherited cores.
The latter were not analysed and no systematic age variations with respect to different cathodolu-
minescence images were found among the others. However, analyses four points made close to
crystal edges tend to yield somewhat lower ages than analyses from the central parts. This trend is
visible in the asymmetrical relative probability plot Fig. 7B.
Regression through ten points analysed in cen- tral parts of the crystals yield an age of 1849 9 14
Ma with an MSWD value of 0.58 Fig. 7B. Some spots are reversed discordant, a phenomenon,
which is not uncommon in analyses made by the SIMS technique. This could be an instrumental
artefact, but micro-scale heterogeneities, with lead gain or uranium loss in the analysed part of the
crystals, cannot be excluded.
The zircon age overlaps with the titanite age of the undeformed granitoid. It is interpreted to
reflect the minimum magmatic age of the pro- tolith, since some lead loss could have occurred
during the subsequent shearing and later reactiva- tions. These events are most likely the reason for
the younger overprintinglead loss in the outer parts of the crystals.
5. Discussion
