417
in one drilling. Within the dierent calc-alkaline plutonic units,
various types of enclaves and dykes occur. Several episodes of Petrography and crystallization sequence
hydrothermal alteration with complex fracture ®llings have been
recognized in the entire batholith )Cathelineau et al. 1999).
of Ca±Al silicates
Secondary Ca±Al silicates occur almost exclusively
within primary magmatic biotite that mostly appears to
be unaltered under optical and electron microscope )e.g.
BSE mode). Pumpellyite was rarely observed within
hornblende, and laumontite mostly occurs in fractures.
Most frequently, the Ca±Al silicates form elongated
lenses parallel to the )001)-cleavage of biotite with a
rather sharp contact to the host biotite. However, ®n-
gering into the interstitial spaces between biotite sheets is
common )Fig. 4b, e, f). In some cases, the host biotite
Fichtelgebirge
The Fichtelgebirge plutonic complex is located at the NW margin
of the Bohemian Massif in SE Germany )Fig. 1b) and is also part of
the Hercynian orogen. The Fichtelgebirge low-to mediumt-em-
perature and low-pressure metasedimentary basement )Mielke
et al. 1979) was intruded by multiphase late-Hercynian granites,
which can be divided into an older )OIC) and a younger intrusive
complex )YIC; Richter and Stettner 1979). The OIC shows radio-
metric ages of ꢀ 326 Ma, whereas the YIC ages range from 305 to
290 Ma )Besang et al. 1976; Carl and Wendt 1993).
Calc-alkaline granodiorites to gabbros occur spatially and are sheets are deformed around the secondary Ca±Al silicate
probably genetically related to the porphyritic biotite±granite of the
grains; this re¯ects mineral growth without biotite re-
OIC. Age dating of these intermediate plutonic rocks revealed very
placement. However, in other cases Ca±Al silicates
dierent ages ranging from ꢀ468 )Rb±Sr on whole rock) to 327 Ma
)mainly prehnite) may replace parts of the host biotite
)K±Ar on biotite, Holl et al. 1989), therefore the genetic place of
)Fig. 4c).
The complete assemblage hydrogarnet + prehnite +
pumpellyite + epidote minerals was only rarely
observed within one thin section.
Secondary epidote minerals may be associated with
the other Ca±Al silicates, but no speci®c relationship can
be observed between them. Secondary epidote associat-
ed with garnet, prehnite or pumpellyite can be distin-
guished from earlier )magmatic) epidote by its rounded
these rocks is still a matter of debate )see also Siebel et al. 1997).
These intermediate to basic plutonites, also called `redwitzites'
)Troll 1968), form a small massif near the type locality north of
Marktredwitz. In addition, some small bodies of redwitzites are
situated at the northern margin of the Fichtelgebirge granite massif
near Frohnlohe and Groûschloppen )Fig. 1b). The dierent rock
types may show diuse contacts. Redwitzites are locally inter-
spersed with porphyritic biotite granite and exhibit features of
magmatic mingling. The intermediate to basic plutonites are rich in
potassium )high-K calc-alkaline), which is re¯ected by their large
amount of biotite. Multiphase hydrothermal alteration has aected
the Fichtelgebirge plutonic complex during post-magmatic times shape and intense yellow colour. In contrast, light yellow
)Haslam 1994; Hecht et al. 1994; Irber et al. 1997).
to colourless primary epidote predominantly occurs as
euhedral to subhedral crystals )Fig. 4a) or as anhedral
grains in close association with hornblende.
Host rock petrography and geochemistry
Hydrogarnet can be found alone or associated with
prehnite, but only rarely with pumpellyite or epidote
minerals. Hydrogarnet in occurs frequently as inclusion
within prehnite, which is replacing biotite )Fig. 4c).
Rarely hydrogarnet may also be intergrown with
prehnite. Hydrogarnet forms less elongated lenses in
comparison to prehnite or pumpellyite. It is slightly bi-
refringent and shows dark grey interference colours.
Prehnite is the most common Ca±Al silicate and may
be associated with all of the other secondary Ca±Al
phases. Although, in general, prehnite is associated with
hydrogarnet )Fig. 4a) and/or pumpellyite )Fig. 4d),
prehnite may be the only secondary Ca±Al silicate. Lo-
cally, the host biotite is nearly completely replaced by
prehnite )Fig. 4c). Prehnite is colourless to light beige,
has grey to yellow colours with crossed nicols and fre-
quently shows a so-called bow-tie structure )Phillips and
Rickwood 1975). Small inclusions of titanite may be
observed within prehnite.
Pumpellyite mostly occurs alone, but can also be in-
timately intergrown with prehnite )prehnite±pumpellyite
paragenesis; Fig. 4d). Pumpellyite and hydrogarnet
never occur in direct contact. Pumpellyite grains are
mostly very small, have intense green to yellow
pleochroism and anomalous interference colours.
In some samples from Charroux-Civray, laumontite
was identi®ed. It occurs mostly isolated in small frac-
tures, rarely in association with prehnite. Exceptionally,
The plutonic rocks of both localities cover a large range in com-
position, including gabbros, diorites, tonalites, leucotonalites,
monzogabbros, monzodiorites, monzonites, monzogranites to
granodiorites )Tables 1 and 2). The main rock-forming minerals are
plagioclase, rarely preserved clinopyroxene, green hornblende,
biotite, quartz, and potassium feldspar in dierent proportions
according to each rock type. Titanite, magnetite, apatite, zircon,
pyrite, chalcopyrite, and allanite are the main accessory minerals in
the basic to intermediate rocks. Apatite, ilmenite, zircon, monazite
and xenotime are the dominant accessories of the granites. In the
Charroux-Civray area, magmatic anhydrite was observed for the
®rst time within plutonic rocks )Freiberger and Cuney 1999). One
particular characteristic of both complexes is the common occur-
rence of magmatic epidote )besides post-magmatic epidote) with
pistacite contents around 28%. The samples from Fichtelgebirge
are signi®cantly enriched in biotite compared with ordinary inter-
mediate to basic plutonites. Biotite occurs locally in relatively big
sheaf-like sheets. Figure 2 shows the geochemical variation of the
plutonic rocks and illustrates the widespread occurrence of Ca±Al
silicates within almost all rock types from both localities.
Occurrence of primary magmatic minerals and secondary hy-
drogarnet, prehnite, pumpellyite, epidote and laumontite is listed in
Table 3. Note that all listed Ca±Al silicates occur within the given
plutonite type, but not necessarily always together in the same
sample.
Our data con®rm that whole rock CaO contents higher than
1 wt% )Fig. 3) are necessary for the formation of one or several of
the Ca±Al silicates )Tulloch 1979). The amount and types of Ca±Al
silicates that occur in each rock type are highly variable down to
the thin section scale. However, no signi®cant variation in the oc-
currence of Ca±Al silicates depending on the spatial position in the
pluton was recognized.