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Mineralogy and Petrology (2001) 72: 185207
Hornblende thermobarometry of granitoidsfrom the Central
Odenwald (Germany) andtheir implications for the
geotectonicdevelopment of the Odenwald
E. Stein1 and C. Dietl2
1 Institut fur Mineralogie, TU Darmstadt, Federal Republic of
Germany2 Geologisch-Palaontologisches Institut, Universitat
Heidelberg, Federal Republic ofGermany
With 6 Figures
Received July 14, 1999;revised version accepted October 6,
2000
Summary
The three major units of the Bergstrasser Odenwald (Frankenstein
Complex,Flasergranitoid Zone and southern Bergstrasser Odenwald)
are, according to literature,separated by two major shear zones.
The aim of the present paper is to evaluate theimportance of these
sutures by comparing new hornblende geothermobarometry datafrom
five plutons of the Flasergranitoid Zone with published P-T data
from the entireBergstrasser Odenwald. Furthermore radiometric,
geochemical and structural data fromthe literature were also used
for this purpose. Temperatures were calculated with
theamphibole-plagioclase thermometer and range from 600 to 800 C.
Determinations ofthe intrusion depth, using the Al-in-hornblende
barometer show that most plutonsintruded at pressures ranging from
about 4 to 6 kbar (13 to 20 km). These combined datado not allow to
postulate a major suture zone between the Flasergranitoid Zone and
thesouthern Bergstrasser Odenwald, while comparison of similar data
from the Flaser-granitoid Zone and the Frankenstein Complex verify
the importance of this shear zone.Moreover, our P-T data show that
the high temperature low pressure metamorphism inthe Bergstrasser
Odenwald can also be interpreted as contact metamorphism and
notnecessarily as regional metamorphism.
Zusammenfassung
Hornblende-Thermobarometrie an Granitoiden des Mittleren
Odenwaldes (Deutsch-land) und ihre Implikation fur die
geotektonische Entwicklung des Odenwaldes
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Die drei Haupteinheiten des Bergstrasser Odenwaldes
(Frankenstein-Komplex, Flaser-granitoid-Zone und sudlicher
Bergstrasser Odenwald) werden nach der Literatur durchzwei
bedeutende Scherzonen voneinander getrennt. Ziel der vorliegenden
Arbeit ist es,die wirkliche Bedeutung dieser beiden Suturen
herauszuarbeiten. Dazu wurden eigene,neue
Hornblende-Geothermobarometrie-Daten, die an funf Plutonen der
Flasergranitoid-zone ermittelt wurden, mit bereits publizierten
P-T-Daten aus dem gesamtenBergstrasser Odenwald verglichen. Zudem
wurden radiometrische, geochemische undstrukturgeologische
Datensatze aus der Literatur fur diesen Zweck benutzt.
Kristalli-sationstemperaturen wurden mit Hilfe des
Amphibol-Plagioklas-Thermometers errech-net und liegen zwischen 600
und 800 C. Die Bestimmung der Intrusionstiefe mit
demAl-in-Hornblende-Barometer ergab fur die meisten Plutone Drucke
im Bereich von 46 kbar (1320 km). Diese, sowie radiometrische,
geochemische und strukturgeologischeDaten aus der
Flasergranitoid-zone und dem sudlichen Bergstrasser Odenwald
gebenkeinen Hinweis auf eine wichtige Suturzone zwischen diesen
beiden geotektonischenEinheiten, wohingegen der Vergleich ahnlicher
Daten aus der Flasergranitoid-Zone unddem Frankenstein-Komplex die
Bedeutung der Scherzone zwischen diesen beidenEinheiten hervorhebt.
Unsere P-T-Daten zeigen auerdem, da die
Hochtemperatur-Niederdruck-Metamorphose im Bergstrasser Odenwald
nicht notwendigerweise eineRegionalmetamorphose sein mu, sondern
ebenso gut als Kontaktmetamorphoseinterpretiert werden kann.
Regional setting of the Odenwald
Introduction
The Crystalline Odenwald, part of a magmatic arc along the
northern margin of theSaxothuringian zone, is the largest exposure
of crystalline rocks within the so-calledMid-German Crystalline
Rise. To the west it is bound by the Rhine valley, to thenorth by
the Saar-Selke Trough and to the south and east it is covered by
Mesozoicsediments (compare Fig. 3, Stein, this volume). The
Crystalline Odenwald can bedivided geographically and geologically
into two parts: the smaller Bollstein Gneissdome to the east and
the Bergstrasser Odenwald to the west. Both parts have
beeninterpreted as magmatic arcs, the first of pre- to early
Variscan (Altenberger andBesch, 1993), the latter of mid- to late
Variscan age (Henes-Klaiber, 1992; Kreher,1994). They are separated
by a major Variscan shear zone, the so-called OtzbergZone (Hess and
Schmidt, 1989). The Bergstrasser Odenwald, which is the subject
ofthe present paper, consists of ca. 90% calc-alkaline magmatic
rocks, and ca. 10%metasediments forming narrow and distinct, NE-SW
trending belts, which separatethe igneous complexes. Willner et al.
(1991) distinguished three units in theBergstrasser Odenwald: the
Frankenstein Complex in the north (unit 1), the
centralFlasergranitoid Zone (unit 2) and the southern Bergstrasser
Odenwald (unit 3),which, according to several authors (e.g.
Henes-Klaiber, 1992; Krohe, 1994; Altherret al., 1999) are also
separated by two important shear zones. The aim of this studywas to
evaluate the geotectonic importance of the postulated suture zones
byapplying hornblende geothermobarometry to rocks from the
Flasergranitoid Zoneand comparing these data with published P-T
data from the entire BergstrasserOdenwald. Moreover, we included
published radiometric, geochemical andstructural data for this
purpose.
186 E. Stein and C. Dietl
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The magmatic rocks of the Bergstrasser Odenwald
Most of the igneous rocks of the Frankenstein Complex and the
adjacentBergstrasser Odenwald show I-type signatures with a typical
subduction-relatedgeochemistry, i.e. they are characterized by
negative anomalies of Nb, Ta and Ti(Henes-Klaiber, 1992). In
particular, radiometric data point to a different intrusionand
cooling history for the northern and southern parts.
207Pb/206Pb-dating on single
Fig. 1. Geologic map of the Flasergranitoid Zone with the
sampled plutons marked
Hornblende thermobarometry of granitoids from the Central
Odenwald 187
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zircons (362 9 Ma) as well as 40Ar/39Ar-dating on hornblende
(363 7 Ma) andplagioclase (359 3 Ma) of gabbros from the
Frankenstein Complex were inter-preted as intrusion ages with a
very rapid subsequent cooling history (Kirsch et al.,1986). Low
Sr-initials (0.703) indicate a mantle-derived gabbroic magma
(Kirschet al., 1986). Intermediate to felsic plutonic rocks of the
central and southernBergstrasser Odenwald have a different
intrusion history: the oldest data wereobtained from a granodiorite
in the northern part (K/Ar hornblende: 340 Ma; K/Arbiotite: 330-327
Ma). Similar rocks from the central and southern
BergstrasserOdenwald provided very homogeneous results which are,
however, ca. 510 Mayounger (K/Ar hornblende: 330-335 Ma; K/Ar
biotite: 323-325 Ma; Kreuzer andHarre, 1975). According to
geochemical data, these felsic rocks were derived frommetaluminous
crustal protoliths (Altherr et al., 1999).
The metamorphic rocks of the Bergstrasser Odenwald
The magmatic rocks of the study area are seperated by four
narrow (less than 1 kmwide) zones of metamorphic rocks, consisting
of gneisses, micaschists, graphite-bearing quartzites, marbles,
calc-silicate rocks and amphibolites. Characteristicmineral
assemblages with sillimanite, andalusite and cordierite in
metapelitic rocksand wollastonite in metacarbonates indicate a high
temperature low pressureamphibolite facies metamorphism (ca. 34
kbar, 600650 C; Okrusch, 1995). Thisimplies a geothermal gradient
of about 50 C/km. Differences in the metamorphichistory between the
Frankenstein Complex, characterized by anticlockwise P-Tpaths, and
clockwise paths in the rest of the Bergstrasser Odenwald were
firstlydescribed by Willner et al. (1991).
235U/207Pb- and 238U/206Pb-dating of zircon from metasedimentary
rocks of thecentral (336-337 Ma) and southern Bergstrasser Odenwald
(342 Ma, 332 Ma) werelinked with the thermal peak of the regional
metamorphism (Todt et al., 1995). Thesubsequent cooling history has
been derived from K/Ar- and 40Ar/39Ar data ofhornblende (343-335
Ma; 334 Ma) and biotite (328-317 Ma; 330 Ma) by Kreuzerand Harre
(1975) and Rittmann (1984).
The geology of the Flasergranitoid Zone
A zone of special interest is the so-called Flasergranitoid Zone
in the centralBergstrasser Odenwald, which is characterized by an
intimate association ofgabbros, diorites, granodiorites and
granites. In the northern part of the Flaser-granitoid Zone
predominantly felsic granites alternating with metasediments
areexposed, whereas in the south basic diorites with a few gabbros
make up ca. 60% ofthe rocks (Stein, 2000). Moreover, biotite
diorites are common in the north, whilehornblende diorites are
restricted to the Hauptdioritzug in the south. Therefore, itcan be
stated that the basicity of the magmatic rocks within the
Flasergranitoid Zonedecreases continuously from south to north.
This trend is reverse to the general trendin the Bergstrasser
Odenwald (Stein, 2000).
Most of the magmatic rocks show a pronounced planar fabric, the
origin ofwhich is still debated. It is either tectonic, due to
syntectonic intrusion in a trans-tensional regime (Krohe, 1994), or
magmatic, due to the successive emplacement of
188 E. Stein and C. Dietl
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different plutons, called nested diapirs (Stein, 2000). Another
obvious trend isdocumented for the fabric development. To the
south, where basic igneous rocks areintimately associated with
intermediate and felsic intrusives as well as metamorphicrocks, the
fabrics are clearly magmatic. Magmatic layering, magmatic
foliations andmagmatic lineations are common. These fabrics are
only locally overprinted bysolid-state deformations (Stein, 2000).
In the northern part, however, magmaticfabrics within granitic
lithologies are strongly obliterated by solid-state fabrics,these
are concentrated in discrete ductile shear zones, which are up to
several metreswide. These shear zones are restricted to a 1.5 km
wide area along the border to theFrankenstein Complex and show all
possible transitions from mylonites to ultra-mylonites. Sinistral
and dextral transport directions were observed next to eachother
(Stein, 2000). Sinistral strike slip zones are well-known from all
over theBergstrasser Odenwald and are described in detail from the
Melibocus Massive(Altenberger et al., this volume). The dextral
ones are restricted to the north.
Willner et al. (1991) and Krohe (1994) describe an important
strike-slip zonebetween the Flasergranitoid Zone and the adjacent
Weschnitz Pluton to the south.Therefore, they divided the central
and the southern part of the BergstrasserOdenwald into two
independent tectono-metamorphic units (unit 2 and unit3), although
the structural and P-T data, obtained from both parts, do
notsignificantly differ. Henes-Klaiber (1992) used this
interpretation of the BergstrasserOdenwald in her study to propose
a continuous increase of the intrusion depth fromthe central toward
the southern Bergstrasser Odenwald. She suggested that theplutons,
which were intruded into different crustal levels, were juxtaposed
along amajor shear zone between the Flasergranitoid Zone and the
southern BergstrasserOdenwald with a considerable vertical
displacement. At first glance the emplace-ment mechanisms seem to
be very different to the north and south of this shearzone. In the
southern unit 3 the calc-alkaline intrusions occur as large,
distinctdiapiric plutons such as the granodiorite of the Weschnitz
Pluton and the granites ofTromm and Heidelberg, whereas in the
Flasergranitoid Zone (unit 2) mostplutons are small, have an
elliptical shape and are intimately associated with eachother
(Stein, 2000).
A brief classification of the intrusives of the Flasergranitoid
Zone
In the Flasergranitoid Zone of the central Bergstrasser Odenwald
Stein (2000)distinguished at least four different types of
intrusions; all of them were sampled:
(1) Round to elliptical plutons with homogeneous and distinctive
lithologies. Theyare characterized by euhedral K-feldspar or
plagioclase phenocrysts, which together with the matrix minerals
and microgranular enclaves are alignedwithin a magmatic foliation
or banding. Quartz is the only mineral with atypical solid-state
deformation imprint (Dietl and Stein, this volume). Typicalexamples
are the Melibocus Granodiorite, and the smaller Ludwigshohe
Granite.
(2) Round to elliptical plutons with a concentric structure.
They show a normalzoning with diorites at the rims and granites in
the cores. The intensity ofmagmatic fabrics within these plutons
increases from the centre towards themargin. The most penetrative
fabrics occur at lithological boundaries anddecrease continuously
toward the centres of the intrusions. This is also true for
Hornblende thermobarometry of granitoids from the Central
Odenwald 189
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solid-state deformations, so that the older diorites at the rim
are strongestoverprinted by solid-state fabrics, whereas the
younger granites in the core inmost cases do not show any
solid-state deformation. Such configurations areexplained by
forceful ballooning. One of the best examples in the
Flaser-granitoid Zone is the Billings Pluton.
(3) Ballooning plutons with a clear reverse zonation, with
undeformed diorites intheir cores and granites with intense
magmatic fabrics and solid-state overprintat their margins. This
intrusion mechanism characterizes nested diapirs, whichare typical
of the southern part of the Flasergranitoid Zone. Good examples
areexposed at the Seemann quarry in Hochstadten or come from the
boundaryregion to the Weschnitz Pluton, where samples A9 and P53
were taken.
(4) Dykes, of dioritic, granodioritic and granitic composition.
Most of them arefine-grained, but porphyritic dykes with
hornblende, plagioclase or K-feldsparphenocrysts are also described
(Nickel and Fettel, 1985). Sample T228IV comesfrom a granodioritic
porphyry dyke.
Sample description
Sample T228IV comes from a granodioritic porphyry dyke from the
northernFlasergranitoid Zone. It shows abundant micrographic
intergrowths of quartz andK-feldspar. Furthermore, this
granodiorite is characterized by several cm-largeplagioclase
phenocrysts and myrmekites. The complete assemblage contains
quartz,K-feldspar, plagioclase (An2833), green hornblende, Ti-rich
biotite, titaniteilmenite, apatite and zircon.
The Ludwigshohe Granite is a light, medium to coarse-grained,
porphyriticgranite with ca. 2 cm large euhedral K-feldspar
phenocrysts. Furthermore it con-tains schlieren and dark,
fine-grained, microdioritic enclaves with dioritic com-position.
The granite consists of the assemblage plagioclase (An25) (38
vol.-%),quartz (23 vol.-%), K-feldspar (21 vol.-%), biotite (12
vol.-%), hornblende (5 vol.-%)and the accessories epidote,
ilmenite, magnetite, hematite, titanite, zircon andmonazite. In the
northern part of the Flasergranitoid Zone Stein (2000)
observedmagmatic fabrics, which are almost obliterated by
successive solid-state strike-slipdeformation.
The Billings Pluton consists of four lithologies: diorite,
granodiorite, granite andporphyry. The diorite is the only rock
type that contains the required mineralassemblage for
geothermobarometry (see below). It is characterized by an
intense,steeply to the NW dipping magmatic foliation with an only
weak solid-stateoverprint. The diorite is medium- to coarse-grained
with large euhedral plagioclaseand hornblende, which are aligned
within the foliation plane. The diorite consistsof plagioclase
(An2540) (6065 vol.-%), hornblende (15 vol.-%), biotite (10
vol.-%), K-feldspar, quartz and chlorite (5 vol.-% each) and
zircon, ilmenite, hematite,calcite and titanite.
Sample A9 comes from a Flasergranodiorite north of the
Hauptdioritzug. Itshows a strong magmatic foliation, which is
steeply inclined (318/51). Thegranodiorite consists of plagioclase
(An2540) (35 vol.-%), quartz (30 vol.-%),hornblende (15 vol.-%),
biotite (10 vol.-%), K-feldspar, and chlorite (5 vol.-% each)and
zircon, ilmenite, hematite, calcite and titanite.
190 E. Stein and C. Dietl
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Sample P53 is a granodiorite from the southernmost edge of the
FlasergranitoidZone, where it is associated with the metamorphic
rocks of the Silbergrubenkopfarea. It consists of plagioclase
(An2530) (35 vol.-%), hornblende (25 vol.-%), quartz(20 vol.-%),
biotite (10 vol.-%), K-feldspar, and chlorite (5 vol.-% each) and
zircon,ilmenite, hematite, apatite and titanite.
Amphiboles as thermobarometric index minerals
All described samples contain mineral assemblages with a certain
amount ofamphiboles, which can be used as good pressure and
temperature indicators inigneous rocks. Therefore several plutons
of the Flasergranitoid Zone were sampled,to gain insight into the
magmatic history of the Central Odenwald area.
Care was taken to examine only unzoned and unaltered amphiboles
of a clearmagmatic origin to get intrusion-related P-T data.
Nomenclature of amphiboles
Amphiboles have been classified according to Leake et al.
(1997). Mineral formulacalculations are based on 23 oxygens,
standardized on 13 cations (without Ca, Naand K).
All the investigated amphiboles plot in the field of calcic
amphiboles, which isdefined by
P(CaNa) on M4 1.00 with Na< 0.50, and Ca 1.50 on M4
(Leake et al., 1997). Within the calcic amphiboles Leake et al.
(1997) havedistinguished 4 groups:
a Na KA 0:50 and Ti< 0:50;b Na KA 0:50 and Ti 0:50;c Na KA
< 0:50 and CaA < 0:50;d Na KA < 0:50 and CaA 0:50:
The amphiboles of the examined plutons belong either to group a)
or c)with a distinct relation to individual plutons and lithologies
(see also Fig. 2): Theporphyritic Ludwighshohe Granite as well as
its microdioritic enclaves for the mostpart contain hastingsites
accompanied by a small number of ferroedenites, ferro-pargasites
and ferrohornblendes in the granite, and some ferroedenites,
magnesio-hastingsites, magnesio- and ferrohornblendes in the
enclaves. All hornblendes of thediorites and granodiorites from the
Billings quarry are magnesiohornblendes. Mostof the amphiboles from
sample A9 are ferrohornblendes, but also some ferro-tschermakites
and one tschermakite occur. Hornblendes in sample P53
havemagnesiohornblenditic composition, but also edenites,
pargasites and magnesio-hastingsites occur. In the granodioritic
porphyry (sample T228IV) mainly magne-siohornblendes are found,
together with edenites and magnesiohastingsites.
Calcic amphiboles are typical for I-type intrusives (Chappel and
White, 1974;Wyborn et al., 1981; White and Chappel, 1983; Clemens
and Wall, 1984), supportingthe results of Henes-Klaiber (1992).
In the following the terms amphibole and hornblende will be
usedsynonymously.
Hornblende thermobarometry of granitoids from the Central
Odenwald 191
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Thermobarometry
Amphiboles are the most useable minerals for geothermobarometry
in calc-alkalineigneous rocks, because they occur in nearly all
calc-alkaline intrusives, regardless ofmafic, intermediate or
felsic compositions. They are stable over a wide P-T rangefrom 123
kbar and 4001150 C (Blundy and Holland, 1990). Many
geothermo-barometers are based on the Al-content of hornblende: The
Al-in-hornblendebarometer (Hammarstrm and Zen, 1986; Hollister et
al., 1987; Johnson and
Fig. 2. Classification of amphiboles according to the
nomenclature of Leake et al. (1997),LuHo Ludwigshohe
192 E. Stein and C. Dietl
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Rutherford, 1989; Thomas and Ernst, 1990; Schmidt, 1992;
Anderson and Smith,1995) is controlled by the total Al-content of
hornblende. The amphibole-plagioclase thermometer (Blundy and
Holland, 1990; Holland and Blundy, 1994) isbased on the number of
Si- and Al-cations on the tetraeder positions of amphiboles.
Factors influencing the Al-content of amphiboles
The intensive parameters pressure, temperature, oxygen fugacity,
as well as thewhole rock composition and the coexisting phases
determine the Al-content ofhornblende. According to Hollister et
al. (1987) the tschermak substitutionSi R2 AlIV AlVI is
pressure-sensitive; with increasing pressure the Al-content in the
hornblende lattice increases, too. Other reactions, such as the
edenitesubstitution Si vacA AlIV K NaA, and reactions involving Ti
(e.g.Ti R2 2AlVI and Ti AlIV AlVI Si) are controlled more by
temperaturethan by pressure (Anderson and Smith, 1995): The higher
the temperature, the moreeffective the edenite substitution. This
results in an increasing Al-content ofhornblende.
Besides these important substitutions the oxygen fugacity plays
a decisive role,as it controls the Fe # f Fe=Mg Feg and Fe3=Fe2 Fe3
ratios: Thelower the oxygen fugacity, the more Fe2 is present.
Spear (1981) and Andersonand Smith (1995) classify a Fe # in the
range from 0 to 0.6 as high, between 0.6 and0.8 as medium and up to
1 as low oxygen fugacity. The relationship of substitutionreactions
involving Al and of the oxygen fugacity is based on the fact, that
a lowoxygen fugacity favors the insertion of Fe2 in the hornblende
lattice. A high Fe2/Fe3-ratio preferently favors the substitution
of Mg by Al during the tschermaksubstitution. A low oxygen fugacity
therefore leads to high Al-contents ofhornblende. Therefore
Anderson (1997) recommends just to use hornblendes with aFe # 0:65
for geobarometry. On the other hand, a high oxygen fugacity leads
to apreferred incorporation of Fe3 into the lattice, which
preferably substitutes Al.This can keep the Al-content of
hornblende low. Anderson and Smith (1995)therefore recommend just
to use amphiboles with a Fe3=Fe2 Fe3-ratio 0:25for barometric
analyses. The general disadvantage of both these criteria is, that
theyare just based on stoichiometric calculations and not on direct
measurements of theFe3 and Fe2 contents. Therefore Fe # and Fe3=Fe2
Fe3 ratios cannotstand as the only criteria, determining the oxygen
fugacity.
Possible further objectives may be derived from the presence of
accessoryminerals. According to Ishihara (1977) magnetite-bearing
igneous rocks (so calledmagnetite series) point to crystallization
conditions under a high oxygen fugacity,whereas ilmenite-bearing
ones (ilmenite series) indicate a low oxygen fugacity.Moreover, the
abundance of titanite indicates a high f O2.
Generally it can be concluded, that hornblende crystallizing
under high f O2give better and more reliable geothermobarometry
results than those growing underlow f O2.
Factors influencing the Al-content of the investigated
hornblendes
Both, the Tschermak and the edenite substitution are important
for amphiboles ofthe investigated plutons indicating that both
temperature and pressure have
Hornblende thermobarometry of granitoids from the Central
Odenwald 193
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influenced the compositions of hornblende of plutons from the
FlasergranitoidZone.
The role of the oxygen fugacity during the hornblende
crystallization is muchmore difficult to evaluate. Most of the
investigated samples fulfill the f O2-criteriaof Anderson (1997)
and Anderson and Smith (1995) Fe # 0:65 andFe3=Fe2 Fe3 0:25 only
hornblendes from the Ludwigshohe Plutonand its enclaves do not.
Results from this locality partly show too low Fe3=Fe2 Fe3 ratios
and a too high Fe #.
On the other hand the porphyritic Ludwigshohe Granite is an
example for thevery few magnetite-bearing granitoids of the
Flasergranitoid Zone. This composi-tion indicates a high oxygen
fugacity and therefore crystallization conditions, whichare
suitable for the geobarometric investigations can be assumed.
Generally all othersampled rocks, even some enclaves in the
Ludwigshohe Granite, are characterizedby the occurence of ilmenite
without magnetite as accessory oxide phase with smallamounts of
titanite, i.e. they probably crystallised under low to medium f O2
con-ditions.
Because all the amphiboles of all plutons fulfill at least one
criteria for a highoxygen fugacity, all samples were used for
geothermobarometry.
The amphibole-plagioclase thermometer
General comments
Although the amphibole-plagioclase thermometer is still under
debate, there is noother geothermometer that can be applied to
calc-alkaline igneous rocks. Further-more, according to our
experience, the resulting temperatures correlate very wellwith
independently determined temperatures of metamorphic rocks, e.g.
using thegarnet-biotite thermometer.
Blundy and Holland (1990) and Holland and Blundy (1994)
published threedifferent calibrations of the amphibole-plagioclase
thermometer. Two are based onthe edenite-tremolite reaction:
4 quartz edenite albite tremolite:One is based on the
edenite-richterite reaction:
edenite albite richterite anorthite:Blundy and Holland (1990)
first proposed a very simple, empirical thermometer
on the basis of the edenite-tremolite reaction, which could be
applied only to quartz-bearing, intermediate to felsic igneous
rocks with plagioclase An 0:92 and Si inhornblende 7:8 atoms p.f.u.
This thermometer is calibrated for temperaturesbetween 500 C and
1100 C. It already takes into account, that the Al-content
ofhornblende does not only depend on temperature, but also on
pressure. Thethermometer is described by the following formula:
T 311 K 0:677Pkbar 48:980:0429 0:0083144 ln Si 4
8 Si
XPlagAb
with Si atoms p. f. u. in hornblende, and XAb in plagioclase in
decimal units.
194 E. Stein and C. Dietl
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The formula presented in this paper has already been changed
slightly from theoriginal version, as it has been adopted to the
plagioclase compositions observed inthe investigated igneous rocks,
which range from albite to andesine.
As this calibration resulted in too high temperatures for some
lithologies (e.g.Poli and Schmidt, 1992), Holland and Blundy (1994)
recalibrated the amphibole-plagioclase thermometer. They extended
the data base to all components, whichtake part in the
edenite-tremolite reaction. Moreover, they considered
non-idealityinstead of ideality. These changes enable an
application of this thermometer A(Holland and Blundy, 1994) to
quartz-bearing metabasites. It now is:
T 313 K 76:95 0:79Pkbar 39:4XANa 22:4XAK 41:5 2:89PkbarXM2Al
0:0650 0:0083144 ln 27XAvacX
T1Si X
PlagAb
256XANaXT1Al
!
Additionally they calibrated a second version, thermometer B,
which is based onthe edenite-richterite reaction (Holland and
Blundy, 1994), and which is applicablealso to quartz-free igneous
rocks:
T 313 K 81:44 33:6XM4Na :66:88 2:92PkbarXM2Al 78:5XT1Al
9:4XANa
0:0721 0:0083144 ln 27XM4Na X
T1Si X
PlagAn
64XM4Ca XT1Al X
PlagAb
!
Although these two thermometers can be used for a wide range of
lithologies,they have one main disadvantage; they take too many
components into account,which all influence the calculated
temperature, and which therefore can all act assources of
error.
Again the presented formulae of thermometers A and B have been
changedslightly. They are also adopted to the An-contents of
plagioclase in the investigatedsamples.
Application of the amphibole-plagioclase thermometer to the
Ludwigshoheand Billings Plutons
One imperative prerequisite for the application of all three
thermometers is theavailability of independently determined
pressure data. Pressures determined byWillner et al. (1991) from
outcrops close to the sampled plutons were used for thispurpose
(Table 1). P-T data of these authors from metamorphic wall rocks of
theLudwigshohe and Billings Plutons as well as the Silbergrubenkopf
area (sampleP53) correlate with the intrusion of the plutons.
Pressures from the Felsberg and theHelgengrund localities, close to
the Ludwigshohe Pluton range from 4.4 kbar to4.6 kbar; data from
the Rimdidim outcrop, close to the Billings Pluton, give ca.2.7
kbar. Unfortunately no hornblende-plagioclase pairs of sample P53
wereinvestigated, although at least one pressure value of the
neighbouring Silbergru-benkopf (4.9 kbar) lies on the prograde
branch of the P-T path and seems tocorrelate with the intrusion of
the granitoids (Willner et al., 1991). P-T data fromthe Gadernheim
locality, close to sample A9, represent only the retrograde
Hornblende thermobarometry of granitoids from the Central
Odenwald 195
-
development of originally medium pressure rocks. As no
independent pressuredata of metamorphic rocks from the northern
Flasergranitoid Zone exist, theamphibole-plagioclase thermometer
was not applied to sample T228IV. Therefore,the three different
calibrations of the amphibole-plagioclase thermometer wereapplied
only to the porphyritic Ludwigshohe Granite, its enclaves and the
BillingsPluton.
Temperature data were calculated for individual
hornblende-plagioclase pairs ofthe different samples (4 from the
porphyritic Ludwigshohe Granite, 16 from itsenclaves and 2 from the
Billings Pluton), from which average temperatures for eachlocality
were derived. The resulting temperature range for both investigated
plutonsis presented in Table 1. Average plagioclase and hornblende
compositions are listedin Table 2. Typical structural relationships
between hornblende and plagioclase areshown in Fig. 3.
As obvious in Fig. 4, temperatures calculated with the 1990
thermometer(Blundy and Holland, 1990) are significantly higher than
those computed with both1994 calibrations. Hornblende and cogenetic
plagioclase of the porphyriticLudwigshohe Granite crystallized at
temperatures of about 768 27 C, theenclaves at 787 37 C and the
Billings Pluton at 697 2 C. All temperatures lieabove the wet
granitic solidus.
Using thermometer A (Holland and Blundy, 1994) significantly
lower tempera-tures were determined: for the porphyritic
Ludwigshohe Granite T 704 47 C,for the Ludwigshohe enclaves T 743
24 C and for the Billings PlutonT 643 10 C. Even lower temperatures
were calculated, using thermometer B(Holland and Blundy, 1994): for
the Ludwighshohe Granite T 626 65 C, for itsmicrodioritic enclave T
660 29 C and for the Billings Pluton T 60519 C. Data calculated
with thermometer B (Holland and Blundy, 1994) are belowthe granitic
solidus. These appear not to be reliable, because structural
relationshipsbetween hornblende and plagioclase in the Ludwigshohe
Granite and its enclavesindicate a magmatic origin of both minerals
(Dietl and Stein, this volume).
Table 1. Comparison of P-T data determined by Willner et al.
(1991) and P-T data from thisstudy
Willner et al. (1991) this paper
outcrop P [kbars] T [C] close to outcrop P [kbars] T [C]
T 228 IV 2.94.1 Helgengrund 4.44.6 560660 Ludwigshohe 4.16.2
626787Felsberg 3.84.7 550Rimdidim 2.7 625 Billings 1.93.0
605697Muhlberg quarry 3.0 Kolmbach 700 A 9 4.56.0 Gadernheim 2.54.3
600610Silbergrubenkopf 2.04.9 600625 P 53 4.35.7 Oberhambach 2.7
625
196 E. Stein and C. Dietl
-
The Al-in-hornblende barometer
General comments
Hammarstrm and Zen (1986) were the first to suggest a
relationship between theAltot-content of amphiboles and the
confining pressure, under which amphibolescrystallized. Based on
microprobe measurements of amphiboles from granitoids, for
Table 2. Average hornblende and plagioclase compositions of the
investigated igneous rocks (LH, gLudwigshohe, granite; LH, e
Ludwigshohe, enclave)
Hornblende thermobarometry of granitoids from the Central
Odenwald 197
-
the intrusion depth of which have been calculated independently
at 2 kbar and8 kbar respectively, they formulated a first empirical
geobarometer:
P3 kbars 3:92 5:03AltotHollister et al. (1987) confirmed this
correlation and empirically extended thebarometer to granitoids,
which crystallized at pressures between 4 and 6 kbar. At
Fig. 3. BSE photographs of typical structural and petrographic
relationships amonga hornblende, K-feldspar and quartz in the
porphyritic Ludwigshohe Granite andb hornblende, biotite,
plagioclase, and quartz in its microdioritic enclaves
198 E. Stein and C. Dietl
-
the same time they reduced the error bar of the barometer with
their recalibratedformula:
P1 kbar 4:76 5:64AltotA first experimental calibration of this
barometer was carried out by Johnson andRutherford (1989) at
temperatures between 720 C and 780 C, taking a CO2H2Omixture with
two different compositions (CO2:H2O 50:50 and 75:25) as fluidphase,
to reach pressures between 2 and 8 kbar. Their formula reads as
follows:
P0:5 kbar 3:46 4:23Altot
Fig. 4. Comparison of the crystallization temperatures of
hornblende-plagioclase pairsfrom the Ludwigshohe and Billings
Plutons, respectively. They were determined with thethree
calibrations of the amphibole-plagioclase thermomter: a Holland and
Blundy (1994)thermometer B on the x-axis versus Holland and Blundy
(1994) thermometer A on the y-axis; b Holland and Blundy (1994)
thermometer B on the x-axis versus Blundy and Holland(1990) on the
y-axis. It is clear from these two graphs, that the Blundy and
Holland (1990)calibration provides the highest temperatures and
thermometer B of Holland and Blundy(1994) the lowest, in some cases
even unrealistic low values (below the wet graniticsolidus)
Hornblende thermobarometry of granitoids from the Central
Odenwald 199
-
Thomas and Ernst (1990) carried out further experiments, using a
pure H2Ofluid at 750 C and a pressure range of 6 to 12 kbar. They
achieved similar results asJohnson and Rutherford, at least for the
pressure range between 6 and 8 kbar.
Schmidt (1992) calibrated his experimental barometer at
temperatures between655 C and 700 C under water saturated
conditions in the pressure range from 2.5to 13 kbar. His
Al-in-hornblende barometer reads:
P0:6 kbar 3:01 4:76AltotAll these four calibrations provided
very similar pressures (Fig. 5).
According to the cited authors several prerequisites have to be
fulfilled strictlyfor a correct application of the barometers:
(1) the assemblage quartz, plagioclase, K-feldspar, hornblende,
biotite, titanite andmagnetite/ilmenite has to be present
contemperaneously with melt,
Fig. 5. a The four most importanttemperature-independent
calibra-tions of the Al-in-hornblende-barometer: H & Z 86:
Hammar-strm and Zen (1986); H et al.87: Hollister et al. (1987); J
& R89: Johnson and Rutherford(1989); S 92: Schmidt (1992).b The
calibration of the Al-in-hornblende-barometer by Ander-son and
Smith (1995) applied todifferent temperatures
200 E. Stein and C. Dietl
-
(2) the barometer can be applied only to rocks, which
crystallized in a pressurerange between 2 and 13 kbar,
(3) plagioclase coexisting with hornblende should range between
An25 and An35,(4) hornblende should have crystallized near the
granitic solidus,(5) the Si-activity of the melt must have been 1,
i.e. it must have been SiO2-
saturated, because the Al-content of hornblende is directly
related to its Si-content and therefore also to the Si-activity of
the entire system,
(6) amphibole should coexist with K-feldspar, because its
activity also influencesthe Al-content of hornblende,
(7) due to the last three prerequisites only rims of hornblende
in contact with quartzand/or K-feldspar should be measured.
Taking all these preconditions into account, the Al-content of
hornblende shouldonly be controlled by the pressure dependent
Tschermak substitution and thereforeit can be used as a good
barometer.
Already Blundy and Holland (1990) emphasized, that temperature
plays a moreimportant role for the Al-content of amphiboles as the
above cited authorsconceeded. Anderson and Smith (1995) therefore
presented a new formulation of theAl-in-hornblende barometer, which
considers all the three intensive parameters pressure, temperature
and oxygen fugacity which control the Al content ofhornblendes.
The recalibration of Anderson and Smith (1995) is based on the
Al-in-hornblende barometers of Johnson and Rutherford (1989) and
Schmidt (1992). Theyintroduce a temperature correction term on the
basis of the amphibole-plagioclasethermometer of Blundy and Holland
(1990). The introduction of the temperaturesensitive edenite
substitution to the barometer enables pressure calculations even
forigneous amphiboles, which did not crystallize at or close to the
granitoids solidus.Oxygen fugacity is a new limiting factor in the
Al-in-hornblende barometer ofAnderson and Smith (1995), as they
restricted its application to amphiboles, whichcrystallized at high
f O2. The authors take the Fe # and the Fe
3=Fe3 Fe2ratio as a measure for f O2. They recommend to use only
hornblende with a Fe# 0:65 and a Fe3=Fe3 Fe2 ratio 0:25 for
barometric purposes, becauseall experimental calibrations of their
Al-in-hornblende barometer were carried outunder medium to high
oxygen fugacities. The new formula of Anderson and Smith(1995)
reads as follows:
P0:6 kbar 3:01 4:76Altot T C 675
85
f0:53Altot 0:005294 T C 675g
Application of the Al-in-hornblende barometer to the
investigated plutons
All five introduced calibrations of the Al-in-hornblende
barometer (Hammarstrmand Zen, 1986; Hollister et al., 1987; Johnson
and Rutherford, 1989; Schmidt, 1992;Anderson and Smith, 1995) were
applied to the sampled plutons of the FlasergranitoidZone (Fig. 1).
The calibration of Anderson and Smith (1995) can only be applied
tothe Ludwigshohe and Billings Plutons, because only for these two
granitoids
Hornblende thermobarometry of granitoids from the Central
Odenwald 201
-
plagioclase analyses are available, which are necessary for the
temperature-correction term based on the amphibole-plagiocase
thermometer of Blundy andHolland (1990). The temperature correction
has been carried out, using average tem-peratures for each
individual pluton (Billings; Ludwigshohe Granite and enclaves).
For the investigated plutons the following pressures were
determined (for agraphical compilation of all data see also Fig. 6,
for an overview of the pressureranges see Table 1):
Fig. 6. All five calbrations of the Al-in-hornblende barometer
applied to the six samplefractions: a) Hammarstrm and Zen (1986);
b) Hollister et al. (1987); c) Johnson andRutherford (1989); d)
Schmidt (1992); e) Anderson and Smith (1995)
202 E. Stein and C. Dietl
-
Sample T228IV: 9 amphibole rim measurements from the
granodioritic porphyry ofsample T228IV (for an average analysis see
Table 2) provided an average Altot of1.501 0.106, equivalent to
pressures of 3.6 0.5 kbar (applying the calibration ofHammarstrm
and Zen, 1986), 3.7 0.6 kbar (Hollister et al., 1987), 2.9 0.4
kbar(Johnson and Rutherford, 1989) and 4.1 0.5 kbar (Schmidt,
1992). Because noplagioclase compositions were measured for the
granodioritic porphyry, pressureswere not
temperature-corrected.
Ludwigshohe Pluton: For pressure determination of the
Ludwigshohe Pluton in total17 hornblende measurements could be
used, 13 from the porphyritic granite itselfand 4 from enclaves and
schlieren. The microprobe analyses were obtained fromamphibole
rims, which are in contact either with quartz or with K-feldspar.
Thestructural relationship of hornblende and quartz/K-feldspar is
displayed in Fig. 3a.Average Altot-contents of hornblendes are
1.931 0.222 for the granite and1.936 0.050 for the enclaves.
Consequently, pressures without applying thetemperature correction
term calculated for the Ludwigshohe Pluton range from 4.7to 6.2
kbar. Both sample fractions, the porphyritic granite and the
enclaves gave thesame average pressure for each calibration. The
lowest pressure (4.7 0.9 kbar) iscalculated with the calibration of
Johnson and Rutherford (1989), the highest(6.2 1.1 kbar) with the
calibration of Schmidt (1992). The other two give inter-mediate
values of 5.8 1.1 kbar (Hammarstrm and Zen, 1986) and 6.1 1.3
kbar(Hollister et al., 1987). Since the amphibole-plagioclase
thermometer of Blundy andHolland (1990) yielded temperatures, which
are significantly above the solidus(granite: 768 C, enclaves: 787
C), a temperature correction, according to Andersonand Smith
(1995), seems to be reasonable. This correction generally leads to
lowerpressure values. Values of 4.5 0.9 kbar for the granite and
4.1 0.2 kbar for theenclaves were determined, which correlate with
those computed with the calibrationof Johnson and Rutherford
(1989). This result, obtained with the Anderson andSmith
calibration seems to fit very well, because Johnson and Rutherford
(1989)calibrated their experiments at temperatures between 720 and
780 C, and this isexactly the temperature range, in which
hornblendes of the Ludwigshohe Plutoncrystallized.
Billings Pluton: In the sample from the Billings Pluton 7
measurements ofhornblende rims in contact with quartz were carried
out. The average Altot-contentis 1.259 0.111. Derived pressures
range from 1.9 0.5 kbar (Johnson andRutherford, 1989) to 3.0 0.5
kbar (Schmidt, 1992). Temperature data of ca.697 C indicate that
the amphiboles crystallized well above the solidus. Therefore,the
calibration of Anderson and Smith (1995) with its temperature
correction termwas applied. According to this calibration the
Billings Pluton was intruded under aconfining pressure of 2.8 0.5
kbar.
Samples A9 and P53: For samples A9 and P53 no plagioclase
microprobe analyseswere carried out. Therefore, only the 4
calibrations without a temperature correctionterm were applied to
these samples. In total 7 hornblende measurements of sampleA9 and 8
of P53 fulfill the requirements of the Al-in-hornblende barometry
(for anaverage analysis see Table 2). Both provided similar average
Altot-values with
Hornblende thermobarometry of granitoids from the Central
Odenwald 203
-
1.889 0.151 for A9 and 1.839 0.042 for P53, resulting in similar
pressuredata. The pressures range from 5.6 0.8 to 6.0 0.7 kbar for
sample A9 andfrom 5.3 0.2 to 5.7 0.2 kbar for sample P53, using
those barometers calibratedat lower temperatures. Only the Johnson
and Rutherford (1989) barometergives lower pressures at 4.5 0.6
kbar for sample A9 and 4.3 0.2 kbar for sampleP53.
Implication of the thermobarometric results for the importanceof
two major shear zones in the Bergstrasser Odenwald
Determinations of the intrusion depth of several plutons in the
Flasergranitoid Zoneusing the Al-in-hornblende geobarometer show
that the analyzed plutons intruded atpressures between ca. 2.5 and
6 kbar, what correlates with intrusion depths of 8 to19.5 km.
Neither a regional distribution pattern within the Flasergranitoid
Zone (e.g.an increasing intrusion depth from north to south), nor
any correlations with thedifferent intrusion types and mechanisms
can be derived from these data.
Samples from the southernmost edge of the Flasergranitoid Zone
(sample P53)and the Weschnitz Pluton (samples WP20 and 26;
Henes-Klaiber, 1992) at thenorthern edge of the southern
Bergstrasser Odenwald provided very similar pressuredata, between
5.0 and 5.7 kbar. Such similar data do not allow to suggest
thatvertical displacement between the Flasergranitoid Zone and the
southernBergstrasser Odenwald took place as proposed by
Henes-Klaiber (1992): Thisinterpretation is supported by several
other arguments (Stein, 2000):
(1) Radiometric data of Kreuzer and Harre (1975), Rittmann
(1984) and Todt et al.(1995) do not indicate either a hiatus
between the intrusions of the central andsouthern Bergstrasser
Odenwald, respectively, or between the metamorphicimprint in both
these units.
(2) No major difference in the geochemical signatures of plutons
from the centraland southern Bergstrasser Odenwald was reported
(Altherr et al., 1999).
(3) Magmatic fabrics in the southernmost Flasergranitoid Zone do
not show anypervasive overprint by solid state deformation as it is
expected in a major shearzone.
(4) The different sizes of the plutons in the southern
Bergstrasser Odenwald andtheir homogeneity compared to the
Flasergranitoid Zone cannot be used asstrong argument for a large
vertical displacement at the boundary, because alsoin the
Flasergranitoid Zone large homogeneous plutons, e.g. the
MelibocusGranodiorite, occur.
On the other hand our P-T data, in agreement with those of
Willner et al. (1991),clearly establish different intrusion depths
for the Frankenstein Gabbro and for theplutons of the
Flasergranitoid Zone. Furthermore radiometric, geochemical
andstructural data again point to important differences between the
FrankensteinComplex and the Flasergranitoid Zone:
(1) The Frankenstein gabbro intruded ca. 360 Ma ago (Kirsch et
al., 1986), that isabout 20 Ma before pluton emplacement in the
Flasergranitoid Zone started(Kreuzer and Harre, 1975).
204 E. Stein and C. Dietl
-
(2) The Frankenstein Gabbro is derived of mantel melts (Kirsch
et al., 1986), whileall plutons in the southern part of the
Bergstrasser Odenwald have a crustalsignature (Altherr et al.,
1999).
(3) Solid state fabrics transpose earlier magmatic fabrics
within the felsic granitesof the northern Flasergranitoid Zone
(Stein, 2000).
All these facts support the model of a major tectonic boundary,
probablydeveloped as strike-slip shear zone with a strong vertical
component between thesetwo units.
Moreover, the results of this study indicate that care must be
taken with theinterpretation of the pervasive high temperaturelow
pressure metamorphism in theBergstrasser Odenwald. This metamorphic
event is generally regarded as regionalmetamorphism and not as
contact metamorphism (Taborszky et al., 1975 andreferences
therein). However, our P-T data for igneous rocks of the
FlasergranitoidZone are very close to the P-T data of Willner et
al. (1991) for metamorphic rocks ofthis zone (Table 1). Considering
that 90% of the entire Bergstrasser Odenwaldconsist of plutonic
rocks and only 10% are made up of metamorphic country rocks,it
appears possible to interpret the high temperaturelow pressure
metamorphism asdynamic contact-metamorphism due to the widespread
intrusions in the BergstrasserOdenwald.
Regarding the entire Crystalline Odenwald, we favour the
interpretation of Stein(2000), who separated the Odenwald into
three main geotectonic units: TheBollstein Odenwald, the
Frankenstein Complex and the Bergstrasser Odenwald. Allthese
different units may have had a common sedimentation and early
tectono-metamorphic development related to a medium pressure
metamorphic event atabout 400-375 Ma. Afterwards these units had
been separated and experienceddifferent tectono-magmatic and
tectono-metamorphic histories. Finally they werejuxtaposed again
along two major strike-slip zones: one between the
FrankensteinComplex and the Flasergranitoid Zone, and the second,
the so-called Otzberg Zone,between the Bollstein and the
Bergstrasser Odenwald.
Acknowledgements
This paper forms part of the habilitation thesis of E. Stein.
This research was financed byDeutsche Forschungsgemeinschaft (DFG)
grants STE 678-1 and STE 678-2. Microprobeanalyses were carried out
at the Material Sciences Department of the TU Darmstadt andguided
by Dr. S. Weinbruch and S. Riedel. We thank Prof. Dr. P. Blumel,
Dr. J. Reinhardtand Dr. D. Scheuvens for constructive discussions.
We owe special thanks to Dr. D. Tannerfor improving the English of
the manuscript, and to Prof. Dr. W. Schubert for his
veryconstructive review.
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Authors addresses: Dr. E. Stein, Institut fur Mineralogie, TU
Darmstadt, Schnittspahn-strasse 9, D-64287 Darmstadt, Federal
Republic of Germany; e-mail: [email protected]; C. Dietl,
Geologisch-Palaontologisches Institut, Universitat Heidelberg,
ImNeuenheimer Feld 234, D-69120 Heidelberg, Federal Republic of
Germany
Hornblende thermobarometry of granitoids from the Central
Odenwald 207
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