W. Siebel ( ) GeoForschungsZentrum, Telegrafenberg D-14473 Potsdam, Germany Fax:#0331 288 1645 E-mail: siebel@gfz-potsdam.de R. Trzebski Institut fu¨r Geologie und Dynamik der Lithospha¨re, Goldschmidtstr. 3, D-37077 Go¨ttingen, Germany G. Stettner Rossteinstr. 8, D-83607 Holzkirchen, Germany L. Hecht Lehrstuhl fu¨r Angewandte Mineralogie und Geochemie, Technische Universita¨t Mu¨nchen, Lichtenbergstr. 4, D-85747 Garching, Germany U. Casten Institut fu¨r Geophysik, Ruhr-Universita¨t, Universita¨tsstr. 150, D-44801 Bochum, Germany A. Ho¨hndorf Bundesanstalt fu¨r Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany P. Mu¨ller Martin-Mu¨ller Str. 16, D-30900 Wedemark, Germany Geol Rundsch (1997) 86, Suppl.: S45—S63 ( Springer-Verlag 1997 ORIGINAL PAPER W. Siebel · R. Trzebski · G. Stettner · L. Hecht U. Casten · A. Ho ¨ hndorf · P. Mu ¨ ller Granitoid magmatism of the NW Bohemian massif revealed: gravity data, composition, age relations and phase concept Received: 16 June 1996 / Accepted: 24 November 1996 Abstract The late Variscan granitoids of the NW Bo- hemian massif (northeast Bavaria, west Bohemia) con- stitute four partly contiguous granitoid complexes: Fichtelgebirge, northern Oberpfalz, Waidhaus-Roz- vadov and Bor, incorporating more than 20 intrusive units. Based on gravity data, the granites can be mo- deled as steeply inclined slab- and wedge-like bodies with thicknesses between 2 and 8 km. A rough estimate of the total volume of the granites is approximately 18 000 km3. Within the four areas named above, com- position ranges from less evolved dioritic rocks, known as the redwitzite suite, to highly evolved gran- ites. The redwitzites comprise metaluminous rocks with dominant I type features. These rocks yield aberrantly old Rb—Sr ages (545—415 Ma), low initial Sr ratios (0.706—0.708) and high and variable e N$ (T) values (1 to — 4). Sr—Nd isotopes of the redwitzites show con- tamination trends towards the granites suggesting mix- ing between mantle magma and crustal granitic melts. An older plutonic association (granites of Bor, Leuch- tenberg, Weissenstadt-Marktleuthen, Zainhammer) is mildly peraluminous, displaying features of both I and S type granitoids. These granites are characterized by Lower Carboniferous ages (Rb—Sr, K—Ar, U—Pb), low to intermediate initial Sr ratios (0.707—0.708) and high e N$ (T) values (—2 to — 4) which overlap with those of paragneisses from the Zone of Erbendorf-Vohenstrauss (ZEV)and from the western part of the Tepla´ Barran- dian. It is postulated that the older granites were for- med either by partial melting of ZEV or Tepla´ Barrandian crust, or alternatively, of preexisting ma- ture crust contaminated by mantle material. The younger granites are strongly peraluminous and of S type. They yield Upper Carboniferous Rb—Sr and K—Ar ages and exhibit a range towards high initial Sr ratios (0.710— 0.720) and low e N$(T) values (— 4 to — 8). Similar values are found in Moldanubian paragneisses and in Saxothuringian metasediments, both of which provide potential source—rock lithologies for these granites. The age and isotope data discussed herein suggest episodic rather than continuous magmatic activity. From a combination of field and analytical data, a three-stage cycle of granitoid intrusion is proposed: (a) a first phase ( & 350— 325 Ma) of two contrasting magma types coexisting in a close spatial context, the redwitzites (phase Ia) and the older granites (phase Ib), (b) a second phase with emplacement ages of 315—310 Ma comprising all younger granites of the northern Oberpfalz and the Waidhaus-Rozvadov com- plex and (c) a third phase with emplacement ages of 305—295 Ma restricted to the Fichtelgebirge. Key words Biotite chemistry · Bohemian massif · Geochemistry · Geochronology · Granitoid magmatism · Gravity data
W. Siebel · R. Trzebski · G. Stettner · L. HechtU. Casten · A. Hohndorf · P. Muller
Granitoid magmatism of the NW Bohemian massif revealed:gravity data, composition, age relations and phase concept
Received: 16 June 1996 / Accepted: 24 November 1996
Abstract The late Variscan granitoids of the NW Bo-hemian massif (northeast Bavaria, west Bohemia) con-stitute four partly contiguous granitoid complexes:Fichtelgebirge, northern Oberpfalz, Waidhaus-Roz-vadov and Bor, incorporating more than 20 intrusiveunits. Based on gravity data, the granites can be mo-deled as steeply inclined slab- and wedge-like bodieswith thicknesses between 2 and 8 km. A rough estimateof the total volume of the granites is approximately18 000 km3. Within the four areas named above, com-position ranges from less evolved dioritic rocks,known as the redwitzite suite, to highly evolved gran-ites. The redwitzites comprise metaluminous rocks withdominant I type features. These rocks yield aberrantlyold Rb—Sr ages (545—415 Ma), low initial Sr ratios(0.706—0.708) and high and variable e
N$(T)values
(1 to —4). Sr—Nd isotopes of the redwitzites show con-tamination trends towards the granites suggesting mix-ing between mantle magma and crustal granitic melts.An older plutonic association (granites of Bor, Leuch-tenberg, Weissenstadt-Marktleuthen, Zainhammer) ismildly peraluminous, displaying features of both I andS type granitoids. These granites are characterized byLower Carboniferous ages (Rb—Sr, K—Ar, U—Pb), lowto intermediate initial Sr ratios (0.707—0.708) and higheN$(T)
values (—2 to —4) which overlap with those ofparagneisses from the Zone of Erbendorf-Vohenstrauss(ZEV) and from the western part of the Tepla Barran-dian. It is postulated that the older granites were for-med either by partial melting of ZEV or TeplaBarrandian crust, or alternatively, of preexisting ma-ture crust contaminated by mantle material. Theyounger granites are strongly peraluminous and ofS type. They yield Upper Carboniferous Rb—Sr andK—Ar ages and exhibit a range towards high initial Srratios (0.710—0.720) and low e
N$(T)values (—4 to —8).
Similar values are found in Moldanubian paragneissesand in Saxothuringian metasediments, both of whichprovide potential source—rock lithologies for thesegranites.
The age and isotope data discussed herein suggestepisodic rather than continuous magmatic activity.From a combination of field and analytical data, athree-stage cycle of granitoid intrusion is proposed:(a) a first phase (&350—325 Ma) of two contrastingmagma types coexisting in a close spatial context, theredwitzites (phase Ia) and the older granites (phase Ib),(b) a second phase with emplacement ages of315—310 Ma comprising all younger granites of thenorthern Oberpfalz and the Waidhaus-Rozvadov com-plex and (c) a third phase with emplacement ages of305—295 Ma restricted to the Fichtelgebirge.
Key words Biotite chemistry · Bohemian massif ·Geochemistry · Geochronology · Granitoidmagmatism · Gravity data
Fig. 1 Simplified map ofnortheast Bavaria and westBohemia showing granitoiddistribution. MR Marktredwitzintrusives (redwitzites and G1granite); R-E Reuth-Erbendorfredwitzites; ¼-I Wurz-Ilsenbach redwitzites; G1R G1Reut granite facies; G1S G1 Selbgranite facies; Ba Barnaugranite; Fa Falkenberg granite;Fl Flossenburg granite; FrFriedenfels granite; ¸bLiebenstein granite; ¸eLeuchtenberg granite; MiMitterteich granite; St Steinwaldgranite; CBG cordierite—biotitegranitoids; ROG Rozvadovgranite; KG Kreuzstein (Kr\ ız\ ovykamen) granite; K¸ Kladrubygranite (? pre-Variscan); G2K,G3K Kosseine granites; ¼BSWest Bohemian shear zone
Introduction
Carboniferous granitoids form an integral part of theMid-European Variscides. In northeast Bavaria andwest Bohemia, granitoids outcrop over approximately1 200 km2. They may be conveniently divided into fourcomplexes, each comprising a number of intrusiveunits: (a) Fichtelgebirge, (b) Northern Oberpfalz in-cluding the Steinwald, (c) Waidhaus-Rozvadov and(d) Bor (Fig. 1). The granitoids intruded into threedistinct tectonometamorphic units, namely the Mol-danubian, the Saxothuringian and the Zone of Erben-dorf-Vohenstrauss (ZEV). The Bor granitoids intrudedalong the West Bohemian shear zone, i.e. between theMoldanubian sensu stricto and the western margin ofthe Tepla Barrandian region. All these granitoids havebeen studied by numerous geologists, and a consider-able database is available to clearly identify their geo-physical, geochemical and isotopic characteristics. Fulldetails of the geophysical aspects can be found in Behret al. (1989), Blız\ kovsky et al. (1975, 1985a, b), Bosumet al. (1994), Bucker (1986), Casten (1990, 1994), Gattoand Casten (1989), Ibrmajer et al. (1989), Plaumann(1986, 1988), Trzebski (1997) and Vigneresse and Corn-well (1994). Regional petrographic and geochemicalcharacteristics of the granitoids are discussed in Breiterand Siebel (1995), Fiala (1980), Fischer (1965), Hecht
(1993, 1994a), Hecht et al. (1993, 1994, 1997), Kohler(1970), Madel (1968, 1975), Maier and Stockhert (1992),Richter and Stettner (1979, 1987), Schodlbauer et al.(1995, 1997), Siebel (1993a), S[ temprok (1992), Stettner(1958), Tavakkoli (1985), Tomas (1971), Troll (1968),Vejnar (1960), Vejnar et al. (1969), Voll (1960) andWendt et al. (1986). Isotopic investigations were car-ried out by Besang et al. (1976), Carl and Wendt (1993),Carl et al. (1989), Friese (1990), Hofmann (1992), Holl(1988), Holl et al. (1989), Kohler and Holzl (1996),Kohler and Muller-Sohnius (1976), Kohler et al. (1974,1989), Lenz (1986), Siebel (1993a, b, 1994, 1995a, b),Siebel et al. (1995, 1996), S[ mejkal (1964), Wendt et al.(1986, 1988, 1992, 1994) and Vejnar (1962). This reviewrepresents an assessment of the northeast Bavarian andwest Bohemian granitoids based chiefly on the resultsof the publications cited above. The main goals are toshow (a) that the granitoids can be subdivided intotemporally distinct phases, partly derived from differ-ent sources, and (b) that crustal sources became in-creasingly important during magmatism.
General characteristics and petrography of the granitoids
The relative intrusion sequence of the northeast Bavar-ian and west Bohemian granitoids was first established
S46
using geological and geochemical relationships (Stet-tner 1958; Fischer 1965; Troll 1968; Tomas 1971; Madel1975; Richter and Stettner 1979, 1987). Based on fieldwork, Fischer (1965) and Troll (1968) deduced that theintermediate redwitzites predate the granites. Combin-ing field and compositional relationships Madel (1975)and Richter and Stettner (1979) argued that the gran-ites of Leuchtenberg and Weissenstadt-Marktleuthen(G1) were older than other granites nearby. Further-more, Vejnar (1962) and Wendt et al. (1988) consideredthe granites of Bor and Zainhammer as early plutons.For the sake of simplicity these four granites are hereinreferred to as older granites to distinguish them fromthe numerous younger plutons.
The granitoids are largely unaffected by late Varis-can tectonic events which involved folding and thesubsequent development of the main faults within andbetween the tectonometamorphic units. Mineral tex-tures that might relate to the Carboniferous deforma-tion are locally developed in the Leuchtenberg and Borgranites (Voll 1960, Krohe et al. 1994, Zulauf 1994).The G1 Reut granite facies crosses the thrust boundaryof the Munchberg massif (Fig. 1), providing evidenceof postkinematic emplacement. The major movementsalong the Erbendorf lineament, which separates Mol-danubian from Saxothuringian crust, predate the em-placement of the Steinwald and Falkenberg granites,both of which crosscut this fault.
The redwitzites comprise multifaceted rock typeswhich range from gabbro to granodiorite, but overall,quartz(monzo)diorite is the most important constitu-ent. Generally, these rocks form relatively small bodiesthat are intimately associated with and crosscut by theolder granites (i.e. Bor, G1, Leuchtenberg, Zainham-mer). Amphibole and biotite are the principal maficphases. Olivine and pyroxene may occur as relict min-erals. Plagioclase composition varies from An
60to
An20
.The older granites comprise coarse-grained por-
phyritic biotite monzogranites and minor grano-diorites, differentiating locally to even-grained,muscovite-rich granites or a sequence of dyke rocks.The Bor pluton consists of early tonalites and quartzdiorites (Bor I), predominantly megacrystic monzo-granites and granodiorites (Bor II) and late vein leuco-monzogranites (Bor III). Petrographic features of theBor I granitoids can be correlated with those of theredwitzites. Hence, in the following sections the Bor-Igranitoids are included in the discussion of the red-witzite suite.
The porphyritic texture of the older granites is for-med by K-feldspar megacrysts which in places showsubparallel flow alignment. Plagioclase compositiongenerally varies from An
30to An
10in early-crystallized
rocks, and from An10
to An2
in rocks crystallized fromthe final melts. Cordierite, altered to pinite, is found inthe Zainhammer granite and is believed to have beenprecipitated from a high-temperature melt (Wendt et al.
1988) or, alternatively, formed by wall-rock assimila-tion (Stettner 1992, p. 117). A magmatic origin was pro-posed for Mn-rich garnet in the Leuchtenberg granite(Siebel 1993a). Different types of mafic to intermediateenclaves become frequent towards the western marginof the G1 and within the G1 Reut granite facies.
The younger granites are heterogeneous, rangingfrom two-mica monzogranites through leucogranitesto geochemically specialized albite—zinnwaldite gran-ites. The rocks are coarse- to fine-grained with hyp-idiomorphic, equigranular and heterogranular textures,the latter containing feldspar megacrysts. With a fewexceptions (Falkenberg, Liebenstein) modal variationwithin the younger granites is limited. Most plutonicunits average 25—35% plagioclase, 20—30% alkali feld-spar and 30—35% quartz. Fractional crystallization ledto a fairly large variation in mineral compositions (seeBiotite geochemistry) and to a gradual increase in themuscovite/biotite ratio in some of these granites. Mi-nor minerals are andalusite, pinite and sillimanite.
According to Stettner (1992), the Falkenberg graniterepresents the central remainder of a sheet-like intru-sion. The northern tongue of the granite is assumed tounderlie Cenozoic sediments of the Mitterteich basinand it may be spatially related to the Marktredwitzintrusions in the north (Fig. 1). The Mitterteich graniteis nested in the central part of this zone, and is thoughtto lie within the porphyritic facies of the Falkenberggranite. The Flossenburg and Barnau granites repres-ent two parts of a coherent intrusional body dipping tothe NE. Both granites show vertical geochemical zona-tion patterns (Tavakkoli 1985). Concentric zonation isa prominent feature in the Steinwald area, where theunits become successively more felsic towards the NWedge (Richter and Stettner 1987). The Kosseine granitesof the Fichtelgebirge are variable-textured rocks. Theycontain different types of enclaves (Schodlbauer et al.1997), high biotite contents and relict minerals (alman-dine-rich garnet, cordierite) suggesting that these rockswere generated in an open magma system involvingcrustal contamination, fractional crystallization and/orrestite unmixing.
Between adjacent granites a development from non-to highly-differentiated granites is well observed (Rich-ter and Stettner 1979, 1987; Tavakkoli 1985). Followingthe succession Falkenberg/Mitterteich, Friedenfels,Steinwald and G2 (‘‘Randgranit’’), G3 (‘‘Kerngranit’’),G4 (Zinngranit), petrographic features argue in favourof multiple emplacement of progressively more felsicmagmas.
Different types of late- and postmagmatic alterationprocesses have been identified within the granites. TheFlossenburg and Steinwald granites show evidence fordecalcification of plagioclase resulting in transforma-tion from monzogranite/leucogranite into two-micaalbite microcline granite (Wendt et al. 1992, 1994). De-silication involving replacement of primary quartz bysericite has been reported by Ernst and von Gehlen (1962)
S47
Fig. 2 Above: Bouguer gravity map of granitoids from the northernOberpfalz and west Bohemia. Below: gravity map displaying themajor gravity anomalies. The lines in the contour map (above)represent the seismic profiles recorded in the study area. FICH¹
Fichtelgebirge; Kyn Kynz\ vart les granite. (For additional abbrevi-ations see Fig. 1)
from a locality within the Falkenberg granite. A similartype of hydrothermal alteration, which results in theformation of episyenites (LaCroix 1920), is locally de-veloped in some of the Fichtelgebirge granites.
Gravity survey
The gravity method is the most appropriate geophysi-cal method for investigations of the geometry and sub-surface extension of the granites. This method utilizesdensity contrasts between the relatively low-densitygranites and the higher-density metamorphic countryrocks. Thus, the granites are commonly characterizedby marked negative anomalies in the gravity field.
The gravity field of the northern Oberpfalz is shownon the Bouguer map of the Oberpfalz on a scale of1 : 200 000 and isoline spacing of 2 mGal (Plaumann1986), and on the Bouguer map of Germany (southernsheet) on a scale of 1 : 500 000 and isoline spacing of1 mGal (Plaumann 1995). In the Oberpfalz the gravitynetwork was essentially enhanced by reconnaissancegeophysical surveys of the KTB location. Supple-mentary gravity measurements were obtained in thenear vicinity of the KTB drill site, producing a high-resolution Bouguer map of the ISO89 area on a scale of1 : 100 000 and isoline spacing of 0.5 mGal (Bosumet al. 1994; Casten 1990, 1994). Granite-related gravitysurveys in the eastern part of the northern Oberpfalzand westernmost Bohemia by Trzebski (1997) alloweda compilation of the Bouguer maps of the Oberpfalzand western Bohemia on a scale of 1 : 200 000 andisoline spacing of 1 mGal. The gravity maps of theCzech Republic (1 : 200 000 and 1 : 500 000) are basedon work performed mainly by the geophysical survey;Geofyzika-Brno (Ibrmajer 1971; Polansky 1973; Blız\ -kovsky et al. 1975, 1985a, b).
The gravity field of the northern Oberpfalz and westBohemia is composed of three major anomalies (Fig. 2):the gravity high or maximum of the ZEV, the minimumof Tirschenreuth and the prominent maximum of theMarianske Lazne\ complex. The gravity highs reflectthe presence of relatively high-density metamorphicrocks, such as mafic and ultra-mafic rocks, amphi-bolites and associated lithologies (Table 1). Conversely,the composite minimum of Tirschenreuth covers largeparts of the granite exposures, regarded as the mainsource of the negative gravity effect. Consequently, themarked gradients separating the positive and negativeanomalies are caused by the high-density contrasts(*o"0.1—0.3]103 kg/m3) between the granites andthe paragneiss/amphibolite suites. Plaumann (1986) as-sumed that the granites of Falkenberg, Friedenfels andSteinwald in the northern Oberpfalz are responsible forthe gravity low of Tirschenreuth. Detailed analyses ofthe negative anomaly show that the low-density sourceis located east of the Falkenberg and Liebenstein
granites, and thus largely extends below the meta-morphic cover of the Zone of Tirschenreuth-Mahring(Trzebski 1997). Wendt et al. (1994) suggest that theFlossenburg granite forms a sheet descending 5—6 kmtowards the northeast. This granite should thereforecontribute to the negative gravity effect. On the otherhand, in the empirical analysis of the gravity field byTrzebski (1996) the gravity low of Tirschenreuth wasresolved into several local anomalies (the gravity lowsof Barnau, Rozvadov, Waidhaus and Bor) each asso-ciated with distinct granites (Fig. 2).
Gravity modeling of the granites in the northernOberpfalz and west Bohemia yields basal depths be-tween 4 and 8 km, and average depths of 6 km (Bosumet al. 1994; Trzebski 1997; Trzebski et al. 1997). Thebasal depths of the older granites (e.g. Bor, Leuchten-berg) show the lowest thicknesses ((4 km). The geo-metry of the Variscan granites in the northern Ober-pfalz was first estimated by Plaumann (1986) as lacco-lithic bodies with average thicknesses of 4—5 km.Bucker (1986) presented similar estimates of shape,
S48
Table 1 Densities of the main lithostratigraphic units in northeastBavaria and west Bohemia
Fig. 3 Example of seismic transparency suggesting the subsurfaceextent of the Falkenberg granite along the KTB 8502-line; after finalstack with automatic line drawing (Trzebski 1996)
depth and volume of the granites and emphasized flat,sheet-like bodies dipping slightly to the east. Signifi-cantly larger granite volumes were postulated by Behret al. (1989) and Behr (1992) where, for instance, thewestern part of the Falkenberg granite extends todepths of more than 6 km. Although the actual shapesof the granites can rarely be modeled without ambi-guity, the gravity data provide a picture of wedge-likegranite bodies standing vertically in the crust. The vol-umes of the present granite bodies in the northern Ober-pfalz and west Bohemia were calculated on the basis ofthe spatial gravity modeling. Volumes of more than15 000 km3 were obtained (see also Trzebski et al. 1997).
The first interpretation of the gravity anomalies re-lated to the Fichtelgebirge was presented by Plaumann(1986, 1988). In the Czech part of the Fichtelgebirge, thedata were based on the gravity map by Polansky andS[ kvor (1975). In 1989 detailed gravity surveys, per-formed by J. L. Vigneresse in collaboration with theUniversity of Munich, covered the Fichtelgebirge gran-ites in the German part providing a station density ofone per square kilometre (Vigneresse and Cornwell1994; Hecht et al. 1997). In contrast to Plaumann(1986), the Bouguer anomaly data were processedwith a reduction density of 2.70]103 kg/m3, instead of2.67]103 kg/m3. The densities of the granites appear
relatively high (2.65]103 kg/m3) compared with thegranites of the northern Oberpfalz (2.61]103 kg/m3).The two major negative anomalies, trending NE—SWand NW—SE, correspond to the older granite of Weis-senstadt-Marktleuthen (G1) and to the younger gran-ites (G2 to G4), respectively, with magnitudes rangingfrom 26 to —36 mGal. The elongation of the G1 graniteanomaly suggests an affinity with the marked gravitylow of the Erzgebirge. In comparison, the anomalousstructure of the G2 to G4 granites suggests a relationwith the Falkenberg, Friedenfels and Steinwald gran-ites to the southeast. Gravity modeling of the G1 gran-ite results in a relatively flat shape with thicknesses of2—3 km in the central part and with a pronounceddeepening towards the eastern flanks to a depth of at least6 km. The G2 to G4 granites have steep flanks extend-ing to a depth of 6—8 km (Hecht et al. 1997). Volumeestimates, including the Czech part, amount to approx-imately 3000 km3 (J. L. Vigneresse, pers. commun.).
Seismic survey
Subsurface three-dimensional modeling of granites us-ing seismic data is hampered because the approachonly roughly resolves interface levels between granitesand surrounding country rocks indicated by higherreflective horizons. Due to their low impedance con-trasts, granitic intrusions are essentially transparent toseismic waves (Jurdy and Phinney 1983; Pratt et al.1985; Matthews 1986).
First geological interpretations of the KTB seismiclines by Weber and Vollbrecht (1989), Vollbrechtet al. (1989) and Franke (1989) presented a quanti-tative line-drawing model of structural units in thecrust of the Oberpfalz. Subsurface shapes of thegranitic bodies were deduced from the contours ofseismic transparency zones (Fig. 3). This approach
S49
0.705
0.710
0.715
0.720
0.725
290300310320330340350
T (Ma)
G4
G2K
G3K
Steinwald
CBGRozvadov
G1a)
b)
Leuchtenberg
Bor
Friedenfels
G2G3
Mitterteich
Falken-berg
( Sr/ Sr)87 86i Younger Granites
Older Granites
Flossen-bürg
G2 in G4
Bärnau
Fig. 4 Rb—Sr dating results of the Variscan granites from northeastBavaria and west Bohemia. The apparent age is plotted against theinitial 87Sr/86Sr ratio with the correlated errors shown as an ellipse.Arrow a: the low initial 87Sr/86Sr ratio of the Friedenfels granite isdifficult to reconcile with the otherwise highly evolved character ofthis granite and its intermediate geochemical position between Fal-kenberg and Steinwald; arrow b: the K—Ar results suggest that thecordierite—biotite granitoids are older than indicated by the appar-ent Rb—Sr whole-rock age (see Fig. 5). Sources — Bor (including BorII monzogranites and granodiorites), cordierite—biotite granitoids,Rozvadov granite: Siebel et al. (1996); Leuchtenberg: Siebel (1995b);G1 (including G1 Selb and G1 Reut granite facies), G2, G2 in G4(inclusions of G2 in G4), G3, G4, G2 and G3 Kosseine: Carl andWendt (1993); Mitterteich: Siebel (1995a); Falkenberg: Wendt et al.(1986); Friedenfels, Steinwald: Wendt et al. (1988, 1992), Barnau,Flossenburg: Wendt et al. (1994)
incorporates only speculative determination of granitebottom depths and thus requires an additional methodto test the lateral extent of the subsurface massdistribution. However, gravity modeling along theseismic KTB/DEKORP profiles by Trzebski (1997)shows that shapes and depth extensions of the granitesestimated from the gravity models are in goodagreement with the contoured seismic transparencyzones.
Intrusion depths and crustal uplift
The granitoids were emplaced as sharply defined bo-dies remote from the zone of magma generation. Mostplutons crop out as NNW- to NW-trending bodies.This general elongation is supposed to reflect deep-seated fracture zones representing the lines of magmainfill. An example for such a zone is the NNW-trendingWest Bohemian shear zone along which several Varis-can granitoids were emplaced (Zulauf 1994; Dorret al. 1996). The granitoids are situated in countryrocks of different metamorphic grades. Contact meta-morphic assemblages around the southern part of theLeuchtenberg granite favour magma emplacement ata depth of approximately 10 km (Okrusch 1971). Thisestimate is in accordance with phengite barometry onsamples of the Falkenberg and Leuchtenberg granites(Maier and Stockhert 1992; Kleemann 1991; Siebel1993a; Zulauf et al. 1997), but it does not hold forall granitoids in this region. Based on analysis ofcontact metamorphic assemblages, Matthes (1951)assumed that the depth of intrusion of the Steinwaldgranite was between 2 and 4 km. As shown for theFichtelgebirge, the older (G1) granites were emplacedat deeper levels than the younger (G2 to G4) granites(Stettner 1958; Richter and Stettner 1979). The G1granite penetrated Cambrian and Ordovician strata atits northern margin suggesting an emplacement level ofless than 10 km. Some of the G2 granites have texturalfeatures suggestive of emplacement as hypabyssal in-trusives (Richter and Stettner 1979). Within the cor-dierite—biotite granitoids of the Waidhaus-Rozvadovpluton, the presence of cordierite would suggest a com-paratively low-pressure emplacement of the magmas(Green 1976).
In the northern Oberpfalz, intrusion depth estimatessuggest a crustal uplift of 6—8 km between early andlate intrusions. In the Fichtelgebirge unroofing duringmagmatism was controlled by uparching and forma-tion of the late Variscan anticline. The intrusion of theG4 granite coincides with the elevation of the deepestcore of the anticline. The available age data and fieldrelationships between the older and younger granitesreinforce the suggestion made by Richter and Stettner(1979) that the uplift of the Fichtelgebirge had reached5—7 km by that time.
Geochronology
All granitic units have been dated by Rb—Sr and K—Armethods (Figs. 4, 5; Table 2) and, to a lesser extent, by40Ar—39Ar, Sm—Nd and U—Pb methods. Holl et al.(1989) found that the 87Rb—87Sr whole-rock system ofthe redwitzites yield apparent ages of 545$16 Ma(Reuth-Erbendorf), 470$33 Ma (Tirschenreuth-Mahring) and 468$9 Ma (Marktredwitz). Theseauthors envisaged a late Variscan age for the redwit-zites, regarding the ‘‘Caledonian’’ ages as mixed ones.A 87Rb—87Sr apparent age of 415$20 Ma was foundfor the Wurz-Ilsenbach redwitzites (Siebel 1994) whichlowers to a late Variscan value when applying three-dimensional regression of the data by the method ofWendt (1993). For the Marktredwitz intrusion, Holl(1988) has found largely inconsistent K—Ar ageswith maximum values of 350 Ma for biotite and344 Ma for amphibole. Amphibole separates from theWurz-Ilsenbach and Reuth-Erbendorf intrusions yieldsimilar 40Ar—39Ar patterns, all showing a high-temper-ature plateau section of 342—346 Ma (Siebel et al. inpreparation). It is open to discussion whether this age
S50
20
4G4
2
2
0
0
4G3
G3 Kösseine
2
2
0
0
4
G2
G2 Kösseine
20
46
G1
20
20
0
20
20
4
Youn
ger
Gra
nite
s
20
Zainhammer
20
Liebenstein
20
468
Falkenberg
20
4Mitterteich
20
4Friedenfels
20
46
Steinwald
20
Flossenbürg
20
Bärnau
20
46
Leuchtenberg
Redwitzites
Bor
Crd.-bi granitoids
Rozvadov
Kreuzstein
290 295 300 305 310 315 320 325 Ma
Old
erG
rani
tes
northern partsouthernpart
Fig. 5 K—Ar age distribution histrogram using 1-Ma time slicesbetween 325 and 290 Ma for northeast Bavarian and west Bohemiangranitoids. Symbols — white: muscovite; black: biotite; hatched:amphibole. Three K—Ar ages of the redwitzites exceed 326 Ma andare not shown. Sources — redwitzites: Holl (1988), Siebel (1993a,1994); Leuchtenberg: Siebel (1995b); Bor II and III, cordierite—bio-tite granitoids, Rozvadov granite, Kreuzstein: Siebel et al. (1996); G1(including G1 Selb and G1 Reut granite facies), G2, G3, G4, G2 andG3 Kosseine: Besang et al. (1976); Carl and Wendt (1993); Mitter-teich: Siebel (1995a); Falkenberg, Liebenstein: Wendt et al. (1986);Friedenfels, Steinwald: Wendt et al. (1988, 1992); Barnau, Flossen-burg: Wendt et al. (1994)
information represents a primary cooling event in thehistory of these rocks.
For the Bor, Leuchtenberg and G1 granites geo-chronological studies suggest late Visean intrusiveevents (stratigraphic terminology used in this work isbased on the time-scale of Odin 1994). The biotite—monzogranite samples of the Bor granite (Bor II) yieldan Rb—Sr whole-rock age of 337$7 Ma (Siebel et al.1996). This age agrees with a U—Pb zircon age (upperintercept in concordia diagram) for this granite of332#11/—5 Ma (Dorr et al. 1996) and provides
an upper limit for the age of crystallization. Forthe Leuchtenberg granite, U—Pb zircon ages of333$5 Ma (Abdullah et al. 1994) and 342$3 (Kohlerand Holzl 1996) were obtained, casting doubt on thelate Visean age previously inferred for this granite(Kohler and Muller-Sohnius 1976; Siebel 1993a). How-ever, late Visean emplacement of the Leuchtenberggranite is supported by K—Ar data from micas ofthe western aureole which were rejuvenated duringcontact metamorphism at approximately 325 Ma (F.Henjes—Kunst, pers. commun.). The G1 granite in-truded at ca. 326 Ma according to Rb—Sr dating (Lenz1986; Carl and Wendt 1993). K—Ar data of muscoviteand biotite indicate that the granites of Bor, Leuchten-berg and G1 cooled down below the argon blockingtemperatures of the micas between 325 and 315 Ma(Fig. 5). Some G1 biotites show evidence of argon loss(Fig. 5) which is explained by later reheating of theyounger granites. A clear-cut K—Ar mineral age pro-gression is apparent within the Wurz-Ilsenbach redwit-zites and the northern half of the Leuchtenberg granite.This was attributed to slow cooling due to delayeduplift or, alternatively, to contact metamorphic over-print by the Falkenberg and Liebenstein granites(Siebel 1995b). K—Ar and 40Ar—39Ar data from theZainhammer granite show thermal disturbances andrange from 317 to 295 Ma (Wendt et al. 1992). Theseages are also related to reheating caused by the intru-sion of the younger granites.
The granites of Barnau, Falkenberg, Flossenburg,Friedenfels and Mitterteich were dated by Rb—Srwhole-rock analyses mainly between 315 and 310 Ma.U—Pb and Pb—Pb data are only available from theFalkenberg granite. Whole-rock and mineral samplesfrom this pluton yielded U—Pb and Pb—Pb ages of307$22 and 316$9 Ma, respectively (Carl et al.1989). Pb—Pb ages obtained by the evaporation tech-nique range widely between 320 and 370 Ma. This wasascribed to the presence of older detrital componentswithin the Falkenberg granite. The Rb—Sr whole-rockages of the Barnau, Falkenberg, Flossenburg, Frieden-fels and Mitterteich granites are essentially equal, with-in analytical uncertainty, to the K—Ar ages ofassociated muscovites. It is a common observation thatthe K—Ar biotite ages tend to be slightly younger thanthe muscovite ages (Fig. 5). This can be explained bythe different blocking temperatures of Ar in both micas,but also by the higher susceptibility of biotite to alter-ation. Muscovite ages from the Flossenburg granite areapproximately 10 Ma younger than the Rb—Sr whole-rock age and the K—Ar muscovite ages found in theadjacent Liebenstein granite. This could be interpretedin terms of different intrusion level, later uplift of theFlossenburg granite compared with the Liebensteingranite, or inhomogeneous temperature distribution inthe crust. Differences in postmagmatic uplift history arealso indicated by discordant K—Ar biotite ages betweenthe Falkenberg granite and the adjacent Mitterteich
S51
Table 2 Synopsis of U—Pb, Rb—Sr, K—Ar age data and Nd model ages (TDM
) for northeast Bavarian and west Bohemian granitoids. Givenuncertainties are 1 p. The uncertainty of the K—Ar mineral data is expressed by employing the 1!p error of the weighted mean. Note that thetotal range is quoted in those cases where the spread of data exceeds the analytical uncertainty of individual measurements
Granitoid U—Pb date Rb—Sr WR K—Ar mineral date (Ma) 87Sr/86Sr(T)
RedwitzitesBor I (4) 317.4$0.9Marktredwitz (13, 14) 468$9 350 to 327 0.7058$0.0009 1.3Reuth-Erbendorf (13, 14) 538$22 308 to 301 0.7043$0.0002 1.3Tirschenreuth (13) 470$33 0.7051$0.0004 1.2Wurz-Ilsenbach (15) 415$20 319 to 307 0.7050$0.0003 1.2
Reference (U—Pb, Rb—Sr, K—Ar data): 1 Besang et al. (1976); 2 Carl and Wendt (1993); 3 Wendt et al. (1994); 4 Siebel et al. (1996); 5 Wendtet al. (1986); 6 Carl et al. (1989); 7 Wendt et al. (1988); 8 Wendt et al. (1992); 9 Siebel (1995a); 10 Dorr et al. (1996); 11 Siebel (1995b); 12 Koh-ler and Holzl (1996); 13 Holl et al. (1989); 14 Holl (1988); 15 Siebel (1994). Nd model ages based on data from Holl et al. (1989) and Siebel et al.(1995)
granite (Fig. 5). For the cordierite—biotite granitoidsand the Kreuzstein (Kr\ ız\ ovy kamen) granite in westBohemia, Rb—Sr whole-rock data suggest ages ofaround 300 Ma (Siebel et al. 1996). In these cases theRb—Sr ages do not date the intrusion because numer-ous K—Ar mica ages define minimum ages of 313 Mafor the cordierite—biotite granitoids and 305 Ma for theKreuzstein granite (Siebel et al. 1996).
A final group of granites which includes G2, G3, G4and the Kosseine granites (G2K, G3K) is characterizedby Stephanian Rb—Sr whole-rock ages, i.e. 305—290 Ma(Carl and Wendt 1993). K—Ar and Rb—Sr mineral agesof the G2 and G3 granites are in the range 292—295 Maand are younger than the Rb—Sr whole-rock ages of305 Ma. This was explained by Carl and Wendt (1993)as a late Stephanian thermal reset caused by the intru-sion of the G4 granite. Paradoxically, the K—Ar ages ofthe G4 granite are slightly older than the apparentRb—Sr whole-rock age, implying a minimum age ofapproximately 300—295 Ma for the G4 intrusion (Fig. 5).Rb—Sr dating of inclusions of G2 granite in G4 granite
(‘‘G2 in G4’’) has yielded a similar age to that of G4(Fig. 4). According to Carl and Wendt (1993), thisindicates the effect of reequilibration of the G2 Rb—Srsystem during the G4 intrusion event. Another ex-planation is given by Hecht (1993), who suggests thatthe G2 granite inclusions can be regarded as thegranophyric roof facies of the G4 granite.
Geochemistry
Whole-rock composition
Approximately 400 samples, covering all intrusions,were used for geochemical evaluation. The rare earthelement distribution of the granites is discussed else-where (Breiter and Siebel 1995; Siebel et al. 1995; Hechtet al. 1997; Irber et al. 1997). In terms of main and traceelement composition, the granitoids form an extendedseries, as shown by the average chemical analyses(Table 3). The redwitzites are metaluminous rocks
S52
Tab
le3a
Ave
rage
anal
yses
ofgr
anitoi
ds
from
nort
heas
tB
avar
iaan
dw
estB
ohe
mia
Gra
nito
idR
edw
itzi
t!R
edw
itzi
t"Zai
nha
mm
erBor
II#
Bor
III$
G1#
G1R
%G
1S&
Leu
chte
nber
g'
Leu
chte
nber
g)
Lie
ben
stei
nFal
ken
ber
gM
itte
rtei
chN
1436
416
1033
922
67
558
SiO
2(w
t%)
52.6
257
.94
68.5
068
.94
73.8
369
.47
73.9
170
.50
74.5
168
.04
70.6
371
.74
TiO
21.
201.
160.
450.
460.
120.
530.
190.
400.
030.
190.
420.
40A
l 2O3
17.0
516
.84
15.5
215
.00
14.1
714
.89
14.4
114
.57
14.3
516
.58
14.4
014
.24
Fe 2O
37.
386.
392.
992.
851.
093.
191.
322.
530.
931.
322.
552.
47M
nO0.
120.
100.
050.
050.
040.
060.
030.
050.
080.
030.
040.
05M
gO6.
163.
900.
630.
960.
300.
900.
340.
670.
070.
370.
530.
51C
aO7.
675.
591.
301.
870.
691.
870.
771.
560.
290.
871.
051.
07N
a 2O2.
813.
013.
303.
413.
593.
433.
593.
274.
103.
443.
232.
91K
2O2.
313.
005.
144.
454.
554.
714.
854.
723.
997.
255.
074.
82P2O
50.
370.
330.
260.
220.
260.
280.
270.
180.
130.
250.
320.
26
Ba
(ppm
)10
2310
1070
987
926
384
435
265
110
693
281
328
Ce
6995
6912
234
9553
103
1843
105
88C
o26
23n.
d.
n.d.
n.d.
n.d.
n.d.
119
1415
11C
r14
011
9n.
d.
4116
118
4235
1110
44C
s8
10n.
d.
914
1019
18n.
d.
n.d.
n.d.
26G
a20
21n.
d.
2118
1923
2735
n.d.
n.d.
27H
f3
2n.
d.
n.d.
n.d.
n.d.
n.d.
32
n.d.
n.d.
2L
a51
5844
7424
6737
515
2851
43N
b19
1814
1511
159
1622
1221
20N
i34
30n.
d.
2419
43
13n.
d.
108
n.d.
Pb
1523
3444
3935
3548
2154
3030
Rb
8612
023
317
020
623
032
121
945
935
335
233
2Sc
2321
78
410
n.d.
73
46
6Sn
n.d.
n.d.
n.d.
711
814
3948
n.d.
2836
Sr58
241
713
223
367
209
7415
75
161
7182
Ta
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
57
83
Th
1522
1534
n.d.
3115
329
1638
33U
34
810
95
66
108
1813
V14
712
923
32n.
d.
5313
30n.
d.
1414
25Y
2222
2420
1017
1114
814
2215
Zn
7378
6556
4260
4754
4142
8462
Zr
231
249
201
228
4922
687
210
3410
019
317
1
!G
abbro
s,di
orites
;"to
nalit
es,g
rano
dio
rite
s;#m
onz
ogra
nite
s,gr
anodi
orites
;$le
uco
monz
ogr
anites
;%R
eutgr
anite
faci
es;&
Sel
bgr
anite
faci
es;'
Bio
tite
gran
ites
;)M
usco
vite
gran
ites
;n.d
.no
tde
term
ined
S53
Tab
le3b
Ave
rage
anal
yses
ofgr
anitoi
ds
from
nort
heas
tB
avar
iaan
dw
estB
ohe
mia
Gra
nito
idFlo
ssen
burg
Bar
nau
Stei
nwal
dFried
enfe
lsC
rd—B
igr
anite
Rozv
adov
Kre
uzs
tein
G2
G3
G2K
G3K
G4
N12
1611
2212
1318
2032
59
36
SiO
2(w
t%)
72.3
273
.19
73.2
173
.56
55.5
173
.61
73.4
274
.25
75.5
474
.24
71.1
675
.09
TiO
20.
140.
130.
060.
140.
320.
110.
010.
230.
160.
240.
500.
07A
l 2O3
14.8
014
.55
15.1
814
.01
25.3
314
.74
15.1
813
.37
13.0
613
.58
14.0
213
.62
Fe 2O
31.
131.
170.
961.
336.
020.
910.
662.
151.
852.
083.
841.
51M
nO0.
030.
030.
050.
030.
080.
030.
090.
040.
030.
030.
050.
03M
gO0.
250.
240.
060.
134.
650.
190.
020.
280.
190.
250.
590.
07C
aO0.
470.
470.
340.
461.
340.
530.
220.
730.
580.
661.
370.
41N
a 2O3.
783.
684.
093.
621.
913.
334.
682.
973.
042.
872.
763.
52K
2O4.
824.
404.
104.
721.
834.
723.
565.
344.
985.
275.
114.
76P2O
50.
360.
350.
430.
340.
380.
360.
510.
190.
190.
230.
260.
23
Ba
(ppm
)13
860
n.d.
8619
118
419
266
133
283
842
37C
en.
d.
n.d.
n.d.
2685
13—
7137
3386
n.d.
Co
n.d.
n.d.
n.d.
n.d.
16n.
d.
n.d.
n.d.
n.d.
n.d.
n.d.
9C
rn.
d.
n.d.
n.d.
n.d.
63n.
d.
n.d.
87
1112
11C
sn.
d.
3491
1611
1659
2633
2212
51G
an.
d.
2840
2345
1927
2221
2116
36H
fn.
d.
n.d.
n.d.
n.d.
32
2n.
d.
n.d.
n.d.
n.d.
n.d.
La
n.d.
1048
5837
7—
4934
3556
37N
b19
1435
179
947
1211
1113
15N
in.
d.
n.d.
n.d.
n.d.
2810
114
410
64
Pb
2121
1925
3940
n.d.
2935
3527
20R
b48
446
477
735
996
235
1095
420
431
371
243
704
Sc4
34
43
n.d.
n.d.
n.d.
6n.
d.
9n.
d.
Sn34
2249
295
1125
1315
115
29Sr
3118
1230
121
505
3929
6092
10Ta
n.d.
n.d.
118
74
25n.
d.
n.d.
n.d.
n.d.
n.d.
Th
1310
58
197
n.d.
2513
1119
10U
127
1718
55
136
105
321
Vn.
d.
n.d.
n.d.
n.d.
38n.
d.
n.d.
119
1331
n.d.
Y11
89
1216
5—
2825
2126
16Z
n64
6783
4917
744
8244
4344
5745
Zr
5852
3257
8955
2011
077
108
257
45
n.d.
not
det
erm
ined
;—no
tdet
ecte
d
S54
Fig. 6 Plot of A/CNK [mol% Al2O
3/(CaO#Na
2O#K
2O)] vs
wt.% SiO2
for northeast Bavarian and west Bohemian granitoids.Inset: data for the cordierite—biotite granitoids
Fig. 7 Rb/Zr vs wt.% TiO2
diagram for northeast Bavarian andwest Bohemian granitoids
Fig. 8 Rb (ppm) vs Nb#Y (ppm) discriminant plot (Pearce et al.1984) for northeast Bavarian and west Bohemian granitoids; syn-CO¸G syn-collision granites; »AG volcanic arc granites, ¼PGwithin-plate granites; ORG ocean-ridge granites
with A/CNK (molecular Al2O
3/Na
2O#K
2O#CaO)
ranging from 0.8 to 1.0 and evolving towards higherA/CNK values with increasing silica (Fig. 6). They be-long to the calc-alkaline magmatic association andhave geochemical signatures of I type granitoids(Chappell and White 1974). The geochemical variationpattern within the redwitzite suite is distinct from thatof the other granitoids. These are peraluminous(A/CNK from 1.0 to 1.3), reflected in the presence ofmuscovite and Al-rich biotite. The enhanced A/CNKmolar ratios of the cordierite—biotite granitoids (insetin Fig. 6) can be ascribed to the high contents ofwell—preserved cordierite (up to 50%). The older gran-ites (Bor, G1, Leuchtenberg, Zainhammer) aretransitional between I and S type, corresponding to themonzogranite— granodiorite group described by Didierand Lameyre (1969). Most of the younger granites areS type.
The Rb/Zr vs TiO2
plot (Fig. 7) can be used todiscriminate between the granitoids. The most notablefeature is the higher TiO
2content of the redwitzites
compared with the other granitoids. With the exceptionof the G1 Selb granite facies, the older granites areroughly lower in Rb than the younger granites. Notethat the cordierite—biotite granitoids and the Rozvadovgranite are depleted in Rb, and therefore plot near theolder granites in this diagram.
Within the plot Rb vs Nb#Y (Fig. 8) the redwitzitesfall within the field of volcanic arc granites (VAG). Theolder granites cross the line separating the VAG fromthe syn-collisional field (syn-COLG), with most ana-lyses plotting in the VAG field. Apart from a fewRb-poor cordierite—biotite granite samples, theyounger granites plot in the syn-COLG field. Nb andY in the Kreuzstein granite tend to be higher than in
the other members, due to the presence of columbite inthese rocks. The total range of data in this diagram isconsistent with the postorogenic nature of the gran-itoids (Pearce et al. 1984). However, the overall shifttowards higher Rb concentration from the redwitzitesthrough the older granites to the younger granitesmay reflect changes in either (a) degree of partialmelting or fractional crystallization, (b) depth ofmagma generation, (c) source-rock type or (e) deutericalteration.
S55
Table 4a Average analyses of biotites from northeast Bavaria and west Bohemia
Fig. 9 AlVI vs Fe/(Fe#Mg) ratio in biotites from northeast Bavar-ian and west Bohemian granitoids (structural formulae calculatedon the basis of 22O)
Biotite composition
Biotite is a very sensitive indicator of physicochemicalconditions and compositional evolution in granitoidgenesis (Speer 1984). Many trace elements such as Ba,Co, Cr, Ni, Rb, V and Zn, are concentrated in biotite(Bailey 1984; Hecht 1993; p. 84), which is present inalmost all granite facies of the study area. Biotite separ-ates from all granites were analysed by X-ray fluores-cence for major and trace elements (data from Hecht1993; Siebel 1993a, 1995a; BGR, unpublished data).FeO, Li and microprobe analyses including F and Clare only available for biotites of the Fichtelgebirgegranites (Hecht 1993; L. Hecht, unpublished data).
Biotites of the redwitzites are Mg-rich (Fig. 9; Table4) and can be classified as Mg biotites according to thescheme of Foster (1960). Based on the genetic classifica-tion of Nachit et al. (1985), the redwitzite samples fall ina high Mg section of the calc-alkaline field which ischaracteristic of rocks containing coexisting biotite andhornblende or pyroxene (Fig. 10). Note that there isalmost no Al enrichment in biotite with increasingdegree of whole-rock differentiation as is seen in theother granitoids. Furthermore, the trace element pat-tern of redwitzite biotites differs significantly from thatof the other granitoids; they display lower Rb and Zncontents, higher Ba contents and very high contents ofcompatible elements such as Co, Cr, Ni and V (Table 4).
The biotites of the older granites have higherFe/(Fe#Mg) ratios and are classified as Fe biotites(Foster 1960), with the exception of some which aredominated by the siderophyllite component. The bio-tites of the more differentiated granites, such as the G1Selb granite facies, are enriched in Li. As exemplified bythe whole-rock data, the biotites of the older granites
S56
Table 4b Average analyses of biotites from northeast Bavaria and west Bohemia
Fig. 10 Granitoid typology based on biotite chemistry (Mg vs Al505
)according to Nachit et al. (1985). Structural formulae of biotitescalculated on the basis of 22O
show a large compositional variation in major andtrace element contents, even within single intrusivecomplexes such as the Leuchtenberg granite (Table 4).Within the older granites significant Al enrichmentin the biotites occurs with increasing differentiation.
This trend is also reflected by a change from the calc-alkaline towards the alumino-potassic field (Fig. 10).
The biotites of the younger granites show a largevariation in major and trace element contents. Withincreasing Fe/(Fe#Mg) ratios, for example, their Alcontents continuously increase towards the siderophyl-lite end member (Fig. 9). In the Fichtelgebirge, Li-richsiderophyllites (G2, G3), or even protolithionites (G4),occur. All of these plot in the alumino-potassic fieldthat is typical for two-mica granites (Fig. 10). In gen-eral, biotites of the younger granites have a higherFe/(Fe#Mg) ratio, lower concentration of Ba, Cr, Niand V, and higher concentration of granitophile ele-ments, such as Cs, Li, Rb and Sn, than the oldergranites (Table 4). The biotites of the cordierite—biotitegranitoids are distinct from the others and are charac-terized by relatively high Al, Cr, Mg, V and Zn at lowBa, Co and Ni contents.
Significant differences in the F and Cl contents andhalogen fugacities have been shown for the biotites ofthe older (G1 and G1 Selb facies ) and younger granites(G2, G3, G4) in the Fichtelgebirge area (Hecht 1993,1994a, b). The younger granites contain biotites withmuch higher halogen contents (max. 4 wt.% F andmax. 0.35 wt.% Cl) than the older granites (max.1 wt.% F and max. 0.1 wt.% Cl). The relative oxygenfugacities were determined from the chemical composi-tion of biotites from the Fichtelgebirge granites (Hecht1993), applying the experimental results of Wones and
S57
Red
witz
ites
Youn
ger
Gra
nite
sO
lder
Gra
nite
s
εNd(T)
Bas
emen
tP
arag
neis
ses
&M
etas
edim
ents
-10 -8 -6 -4 -2 0 2
Moldanubian,Saxothuringian
ZEV, ZTT
Bor I (2)Wurz-Ilsenbach (4)
Tirschenreuth-Mähring (5)
Marktredwitz (11)
Bor II & III (8)
Leuchtenberg (12)
Zainhammer (3)
G1 (10)
Crd.-bi granitoids (3)Rozvadov (3)
Kreuzstein (3)Mitterteich (3)
G4 (4)
G2 & G2K (5)
G3 & G3K (5)
Friedenfels (5)
Flossenbürg (4)
Bärnau (6)
Falkenberg (9)
Steinwald (3)
Liebenstein (8)
Reuth-Erbendorf (5)
** ** *
* ***
* *
** **
****
** * *****
Fig. 11 Compilation of eN$(T)
values for northeast Bavarian andwest Bohemian granitoids. Numbers in parentheses refer to thenumber of analyses. Data sources: Holl et al. (1989), Siebel et al.(1995, 1996). The fields of basement paragneisses and metasedimentsat 320 Ma were derived from unpublished data (F. Henjes-Kunst).This figure is an updated version of the diagram presented in Siebelet al. (1995)
Eugster (1965). The biotites of the older granites giveoxygen fugacities close to the Ni—NiO buffer, whereasthe biotites of the younger granites indicate much loweroxygen fugacities around the quartz—fayalite—magne-tite buffer. Consequently, it is suggested that theyounger granites were formed under more reducingconditions than the older granites.
Sr and Nd isotopes
Low 87Sr/86Sr340M!
ratios ranging from 0.705 to 0.708are confined to the redwitzites. The older granites covera narrow range in 87Sr/86Sr
330M!with ratios of 0.707
and 0.708 (Fig. 4). The younger granites show a widerange of 87Sr/86Sr
310~300M!ratios between 0.708 and
0.720, with most greater than 0.710, displaying a pro-nounced crustal signature. The distinctly lower initialSr ratios of the less evolved Falkenberg, Friedenfels,G2, G3 and Mitterteich granites (0.708—0.710) relativeto the strongly evolved Barnau, Flossenburg andSteinwald granites (0.714—0.719) might suggest that theisotopic composition of the parent magmas changedduring emplacement, as a result of increasing influenceof felsic crustal material and/or protracted time ofdifferentiation.
The initial Nd isotopic ratios show a clear distinctionbetween the older granites, including the redwitzites,and the younger granites (Fig. 11). With few exceptions,the older granites cover a range in e
N$(T)from 0 to —4.
The redwitzite values are more scattered in compari-son. The younger granites have e
N$ (T)values lower than
—4. Two-stage Nd model ages (¹DM
) of the granitoidsare in the range 1.2—1.7 Ga (Table 2). These ages pro-vide support for substantial involvement of old crust inthe generation of the granitic melts.
Phase concept
Geochronological evidence has led to a considerablerefinement of the emplacement sequence of the gran-itoids. Combining all available data, the history ofmagmatic activity can be subdivided into three tem-porally distinct phases (Table 5):
Phase Ia: Intrusion of the redwitzites (&350—325 Ma).Phase Ib: Intrusion of the granites of Bor (Bor II andIII), Weissenstadt-Marktleuthen (G1, including G1Reut and G1 Selb facies), Leuchtenberg and Zainham-mer, i.e. those units which have been referred to as oldergranites (340—325 Ma). Although it is evident from fieldobservations that phase Ib was predated by phase Ia,there is insufficient reliable geochronological data forthe redwitzites to rule out the possibility that the gran-itoids of both phases intruded broadly coevally.Phase II: Intrusion of the granites of Barnau, Falken-berg, Flossenburg, Friedenfels, Kreuzstein, Liebenstein,
Mitterteich, Rozvadov and Steinwald (315—310 Ma).Subordinate, atypical members, such as the cordierite-biotite granitoids of the Waidhaus-Rozvadov pluton,are considered to be part of this group.Phase III: Intrusion of the G2 and G3 granites,the Kosseine granites (G2K, G3K) and the G4
S58
Table 5 Summary of plutonicevents (phase concept) innortheast Bavaria and westBohemia
Biotite- and two-mica monzogranites,minor granodiorites andleucogranites
Bor II, Bor III, G1, Leuchtenberg,Zainhammer
Phase Ia
350—325 MaRedwitzites
Diorites, tonalites, minorgranodiorites and gabbros
Bor I, Marktredwitz, Reuth-Erbendorf,Tirschenreuth-Mahring, Wurz-Ilsenbach
granite. Such rocks are restricted to the Fichtelgebirgecomprising granites where 87Rb—87Sr and 40K—40Arstudies indicate Stephanian ages (305—295 Ma).
In general, isotopic data confirm the intrusion se-quence determined from field and geochemical studies.However, it is obvious that some granites displayingsimilar petrographic and geochemical characteristicsand which could, therefore, be grouped together, showeither differences in age or isotopic composition, fea-tures which do not favour a close relationship [e.g.Leuchtenberg (phase Ib) and Liebenstein (phase II) orKreuzstein (phase II) and G4 (phase III)]. A minimumage of 320 Ma is inferred for the Falkenberg granitefrom thermal modeling of the cooling history (Zulaufet al. 1997). However, an affinity of the Falkenberggranite with phase II is supported by the Sr and Ndisotopic data (Figs. 4, 11). In the Fichtelgebirge theKosseine granites G2K and G3K have similar Rb—Srwhole-rock ages (290 Ma) to the highly differentiated G4granite (Carl and Wendt 1993). However, on the basis oftheir outcrop pattern, the Kosseine granites belong tothe G2/G3 stage (305 Ma) and represent the deepestintrusion level elevated during the G4 magmatic activity.
Controls on mobilization of granitic melts and potentialmagma sources
The importance of the role of late Variscan low-pres-sure high-temperature regional metamorphism in crus-tal melting and magma generation is widely accepted.One of the major unanswered questions concerns thecritical factors giving rise to the existence of differentgranitoid phases. Are the phases primarily the result ofthe differential availability of fertile source rocks, orof magma ascent mechanisms controlled by regional
tectonic episodes? Provided that tectonics played a vi-tal role for granite emplacement, large-scale faultingmay have facilitated the uprise of the granitic melts.The case for regional control of granitic activity isstrengthened by the fact that magmatism was broadlysynchronous within the different regions (Fichtel-gebirge, northern Oberpfalz, Waidhaus-Rozvadov,Bor). Phase-Ia and -Ib granitoids are characterized bylarge compositional differences, although field evidencesuggests a spatial and temporal association of bothphases. As can be seen from field relations, the conduitsor ascent paths which defined the intrusion localities ofthe redwitzites (phase Ia) also influenced the localitiesof the phase-Ib granites (Madel 1968, p. 62). Within theBor complex, phase-Ia (Bor I) and -Ib (Bor II and III)magma accumulation was partly aided by activation oflarge fracture zones such as the West Bohemian shearzone (Krohe et al. 1994; Zulauf 1994). We thereforeenvisage the existence of a single tectonic event for theupward passage of the phase-Ia and -Ib magmas. Themajor change in magmatic activity took place betweenthe older granitoids (phases Ia and b) and the youngergranitoids (phases II and III). This change may beascribed to the tapping of new source compositions(Fig. 11). It seems that from this point onwards, largevolumes of felsic melts were produced by reworking ofmore fertile crustal domains. According to the geo-chronological data, the transition from phase-Ib tophase-II magmatism took place during a period of atleast 10—20 Ma. In sharp contrast to compositionaldifferences between phases I and II, only minor differ-ences are present between the younger granites ofphases II and III. The time gap of approximately5—10 Ma observed between these two phases was prob-ably merely a result of different tectonic events.
Huge quantities of heat are required for magmatism.Hecht et al. (1989), Hecht (1994b) and Siebel et al.
S59
(1995) envisaged that granite magmatism in this regionwas triggered by the influx of mantle magma into thelower crust. Relicts of potential mantle contributionsare represented by the sparse redwitzites and probablyalso by mafic enclaves in the phase-Ib granites. Mantleinput is also indicated by late Variscan granitoids (e.g.Mute\ nın diorite) that intruded further to the south intothe West Bohemian shear zone (Zulauf 1994; Wulf et al.1996). We suggest that underplating of basic intrusionswas responsible for the onset of high-temperature par-tial melting of crustal source lithologies.
We also call attention to the relationship betweenthickened orogenic crust and granite genesis. The maincrustal thickening of the Bohemian massif had alreadyoccurred prior to the Carboniferous, probably duringthe Upper Devonian when Peri-Gondwana was amal-gamated with the East European platform and largeregions of the former oceanic crust had been subducted.A Carboniferous orogenic event causing crustal thick-ening and formation of the Fichtelgebirge-Erzgebirgeanticline has taken place within the southern Saxo-thuringian zone.
Geochemical characteristics show that the redwit-zites differ from the granites by distinct fractionationtrends and compositional gaps. This suggests that thesources of the redwitzites are not related to those of theother phases by simple geochemical fractionation pro-cesses. The wide range of geochemical and isotopiccomposititon of the redwitzites has led to an equallywide range of interpretations concerning their origin(see Troll 1968). Matthes and Richter (1990) suggestedredwitzites could be formed by mixing granitic andbasic melts, the latter of which were apparently formedby anatexis of country rocks. Holl et al. (1989) havechallenged a hybrid origin of the Reuth-Erbendorf,Tirschenreuth-Mahring and Marktredwitz redwitzites,pointing out that their isotopic features can be gener-ated by mixing mantle magma with granitic melt.
The granites of phases Ib, II and III are characterizedby strong chemical fractionation even within singleintrusive units. These magmas were held in residencelong enough for differentiation to occur. The decreasein oxygen fugacities in the biotites from the older to theyounger granites in the Fichtelgebirge implies thatcrustal components containing organic matter becameincreasingly important as potential source rocks. Thesignificantly higher halogen contents in the phase-IIIgranites is consistent with a greater contribution fromrelatively mature crustal rocks. Another explanationfor the variation of halogen contents could be thechange in melting conditions in the crustal source re-gion. The fluids necessary for partial melting can beenriched in halogens, such as fluorine, if they are gener-ated by the breakdown of biotite in the source rock(Christiansen et al. 1988; London 1995). Partial meltingof crustal material preferentially induced by biotitebreakdown was probably an important process duringformation of the phase-II and phase-III granites.
Initial Sr and Nd isotopic ratios are powerful indi-cators of the source of the magmas. The range in Sr andNd isotopic composition found in the phase-Ia gran-itoids could indicate derivation from subductedoceanic crust, or a mantle source with some contamina-tion from old radiogenic continental crust. As men-tioned above, mixing between mantle and crustalcomponents was also favoured by Holl et al. (1989).Based on Sr and Nd ratios, source rocks of phase-Ibgranitoids could be dominated by mafic-rich crustalmaterial or, alternatively, by preexisting mature crustcontaminated by mantle material. High crustal influ-ence seems to be very important for the geneses of thephase-II and phase-III granites and may account forthe Nd isotopic differences between them and the oldergranitoids.
Comparison of granitoid eN$(T)
values with those ofthe metasedimentary basement rocks exposed nearby(F. Henjes-Kunst, unpublished data) allows a closerspecification of possible source materials for the gran-itoids. The basement rocks can be divided into twoisotopically distinct regions: (a) ZEV and Tepla Bar-randian, and (b) Moldanubian and Saxothuringian.Paragneisses of the ZEV and of the Tepla Barrandianexhibit a range in eNd
320M!which encompasses the
phase-Ia and -Ib granitoids. On the other hand,there is a clear overlap in both I
S3and eNd
320M!be-
tween the Moldanubian and Saxothuringian countryrocks and the phase-II and phase-III granitoids(Fig. 11).
Summary and conclusions
Late-Variscan granitoids of northeast Bavaria and westBohemia can be divided into the following major tem-poral phases: a redwitzitic truly I type association(phase Ia), a biotite monzogranite association (granitesof Bor, G1, Leuchtenberg and Zainhammer) showingtransitional I and S type characteristics (phase Ib) andtwo leucogranite associations (phase II: granites of Bar-nau, Falkenberg, Flossenburg, Friedenfels, Kreuzstein,Mitterteich, Steinwald, Rozvadov and phase III: G2,G2K, G3, G3K and G4) with features corresponding totypical S type granites. Gravity data provide informa-tion on deeper structures of the granites. Within thenorthern Oberpfalz and west Bohemia, the granites ofBor, Falkenberg, Flossenburg, Barnau and Waidhaus-Rozvadov yield thicknesses between 4 and 8 km,whereas the granites of the Fichtelgebirge extend to2—8 km below the surface.
If we consider the age and isotopic information,granitoid evolution may be constrained as follows:
Phase-Ia granitoids (granodiorites, tonalites, (quartz)diorites and minor gabbros): 350—325 Ma with lowapparent 87Sr/86Sr
340M!ratios of 0.705 to 0.708 and
eN$(T)
values of 1 to —4
S60
Phase Ib granitoids (granodiorites, monzogranites,leucogranites) : 340—325 Ma with low to intermediateinitial 87Sr/86Sr ratios of 0.707—0.708 and e
N$(T)values
of —2 to —4Phase II granitoids (two-mica monzogranites, leuco-granites): 315—310 Ma with moderate to high initial87Sr/86Sr ratios of 0.710 to 0.719 and e
N$(T)values of —4
to —8Phase III granitoids (two-mica monzogranites, leuco-granites): 305—295 Ma with very high initial 87Sr/86Srratios of 0.716 to 0.720 and e
N$(T)values of —6 to —8
Phase-Ia granitoids represent a relatively small vol-ume of mafic magmas that show signs of hybridization.Both the older (phase Ib) and the younger (phases IIand III) granites comprise homogeneous rocks withstrong compositional zoning indicating intensive frac-tionation culminating in highly evolved leucogranites.The existence of different granitoid phases appears tobe a direct consequence of major tectonic episodes andchanges in source-rock availability. The systematicvariations in chemical and isotopic composition fromphase Ia to phase III are thought to result principallyfrom the increasing influence of crustal source material.Geochemical composition and isotopic characteristicsof the phase-Ia granitoids are interpreted in terms ofinteraction between crust and mantle-derived magmas.Compositional features of the phase Ib granitoids areconsistent with an origin as anatectic melts mainlyfrom preexisting mature crust, with a contribution ofmantle material or, alternatively, as melts of chemicallyless-evolved crust resembling paragneisses of the ZEVand Tepla Barrandian tectonometamorphic units.Phase-II and phase-III granitoids show evidence ofhaving been solely derived from mature crustal seg-ments resembling surrounding Moldanubian para-gneisses and Saxothuringian metasediments.
Acknowledgements We are grateful to Prof. P. Moller, Prof. Dr. I.Wendt, Dr. W. Irber, Dr. G. Zulauf and Jan Lindsay for criticallyreviewing the manuscript.
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