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Constraints on Variscan granite emplacement in north-east Bavaria, Germany: further clues from a petrogenetic study of the Mitterteich granite
Abstract
Petrochemical and Rb-Sr, K-Ar and Sm-Nd isotopic data presented for the Mitterteich granite provide information on whole rock and mineral compositional characteristics, intrusion and cooling history, and protolith nature and put further constraints on the Variscan magmatic evolution in north-east Bavaria.The compositional characteristics classify the Mitterteich granite as a peraluminous (monzo-)granite (SiO2 67.3–73.5 wt.% ). Values for K2O/Na2O (> 1.2 and Al2O3/(CaO + N2O + K2O) (>1.1) are in the range of S-type granites. The rare earth elements show fractionated chondrite-normalized patterns (La
N
/Yb
N
=24–19) with negative Eu anomalies (Eu
N
/Eu
N
*=0.35–0.19). The micas have restricted ranges of major element composition, but reveal notable variations in trace element concentrations. Different biotite fractions of single specimens show a trend to lower concentrations of compatible elements in the finer fraction which can be explained as a result of asynchronous growth during the fractionation process. The PT conditions of crystallization of the magma based on muscovite and biotite is 600–640C at 3 kbar. Regression of the whole rock samples gives an isochron corresponding to a 87Rb-87Sr age of 310 7 Ma, initial 87Sr/86Sr of 0.71040.0010 (2 errors) and MSWD =0.03. Muscovite and biotite yield concordant K-Ar ages between 310 and 308 Ma, indicating a fast cooling rate of the granite intrusion. Nd
T310values average –4.21.0. Nd model ages of 1.4 Ga suggest a source region of mid-Proterozoic age.The Rb-Sr isochron age and initial Sr ratio of the Mitterteich granite are indistinguishable from those of the adjacent Falkenberg granite, establishing a genetic link. However, the K-Ar mica ages suggest that the Mitterteich granite must have undergone a faster uplift or cooling history than Falkenberg. Confronted with the geochronological record of granite emplacement in north-east Bavaria, the new results substantiate the view of three key periods of magmatic activity around 330–325, 315–305 and 290 Ma.
... Chappell, 1999;Chappell and White, 1974, 1992White et al., 1986), Himalayan two-mica and tourmaline leucogranites (e.g. Inger and Harris, 1993;Searle et al., 1997;Visano and Lombardo, 2002), and Variscan twomica granites in Central Europe (e.g., Chen et al., 2003;Liew et al., 1989;Siebel, 1995;Williamson et al., 1996). These are characterized by Fig. 8. CL images of the dated zircons from the Uludağ two-mica granites. ...
... This is in line with the presence of the schlieren- (2008), Altunkaynak et al. (2012) and Aysal (2015), (b) Comparison of the 87 Sr/ 86 Sr (i) and εNd (i) compositions of S-type including the two-mica granites from Uludağ (this study), Himalaya and the Variscan belt in Europe. Data are from Daniel et al. (1987), Inger and Harris (1993), Liew et al. (1989), Siebel (1995), Williamson et al. (1996), andChen et al. (2003). Lachlan Fold Belt S-and I-type granites after McCulloch and Chappell (1982 Hart (1984). ...
... Gardien Fig. 11. Compositional comparison of the Uludağ two-mica granites with the I-and S-type granites from the Lachan Fold Belt (Chappell and White, 2001), Himalayan two-mica and tourmaline granites (Inger and Harris, 1993;Searle et al., 1997;Visano and Lombardo, 2002) and Variscan two-mica granites from Europe (Chen et al., 2003;Liew et al., 1989;Siebel, 1995;Williamson et al., 1996) in terms of (a) Na 2 O versus K 2 O, (b) CaO versus FeO*, and (c) Sr versus Rb. Pattison (1992). ...
This paper deals with the petrogenesis of two-mica granites within the amphibolite-facies metamorphic rocks in the Uludağ Massif (NW Turkey). The two-mica granites are represented by three different types (i) a 17 km long and 1.5 km-wide strongly foliated and lineated metagranite, (ii) a relatively younger circular granite, 11 km by 6.5 km, free of any penetrative fabric, and (iii) up to 5 m thick dikes of unfoliated granite-porphyries. Mineral constituents include quartz, plagioclase, microcline, biotite, muscovite and minor apatite, zircon, ilmenite and monazite. U-Pb zircon dating indicates that the metagranite was emplaced at 35 ± 1 Ma (2σ, latest Eocene) and the unfoliated granite at 28 ± 1 Ma (2σ) (Early-Late Oligocene boundary). Overall, the two-mica granites are characterized by (i) a restricted bulk composition (SiO2 ~ 70-76 wt%), (ii) near-linear to linear inter-element variations, (iii) high concentrations of Al2O3, Sr, Ba, LREEs, and low abundances of Rb, Y, Sc and HREEs, comparable with adakitic rocks, (iv) variably pronounced concave-upward shape of HREEs and absent to feeble Eu anomaly, (v) relatively high Na2O, CaO and Sr, and lower K2O and Rb contents at identical silica contents relative to the classical S-type granites. ⁸⁷Sr/⁸⁶Sr(i) and εNd(i) values range from 0.7052 to 0.7080, and 0.7 to –3.6, respectively. The ²⁰⁶Pb/²⁰⁴Pb(i) values range from 18.23 to 18.69, ²⁰⁷Pb/²⁰⁴Pb(i) from 15.63-15.70 and ²⁰⁸Pb/²⁰⁴Pb(i) from 38.48-38.77. ⁸⁷Sr/⁸⁶Sr(i) and εNd(i), and εNd(i) and ²⁰⁷Pb/²⁰⁴Pb(i) values are negatively correlated. Isotopic variation and negative correlations can best be explained by a heterogeneous source, including an older crustal component and a more juvenile basic component. Magma temperatures are estimated as 754-790 °C based on zircon and monazite saturation. All these features such as (i) relatively Na-, Ca- and Sr-rich compositions, (ii) isotopically heterogeneous source with inferred residual phases such as garnet and hornblende, (iii) occurrence in a ductile strike-slip zone, and (iv) relatively low magma temperatures can be accounted for by water-fluxed melting of a metagreywacke source with substantial mafic component at middle to lower crustal depths.
... In order to determine these endmembers a principal component analysis (PCA) was performed using data from different sources. The data set comprised (1) Ca, Mg, Ba, Na, K, Sr and Rb concentrations and 87 Sr/ 86 Sr ratios of the water samples, digested mineral samples and sequential leachate samples of the study, (2) Ca, Mg, Ba, Na, K, Sr and Rb concentrations of leachates from Irber (1996) and Irber et al. (1997) using samples from the Lehstenbach catchment and granite samples from other sites in the Fichtelgebirge Mountains and in the Ore Mountains (East Germany) and (3) Ca, Mg, Ba, Na, K and Rb concentrations of digested mineral samples from the Bohemian Massif, the Fichtelgebirge mountains and the nearby Mitterteich granites from Siebel (1995) and Siebel et al. (1997). There were no Sr isotope ratios available for these biotite samples. ...
... where c x,norm is the normalised concentration of element x; c x is the concentration of element x; 1 n P n c x is the mean of the concentrations of all n elements from one dataset used for the PCA (i.e. datasets of mineral leaching, water samples, minerals from the present study, the dataset from Irber (1996) and Irber et al. (1997) and the dataset from Siebel (1995) and Siebel et al. (1997)) c x;stand ¼ logðc x;norm Þ À 1 n P n logðcÞ n;m SDðlogðcÞ n;m Þ ð2Þ ...
From a biogeochemical perspective, catchments can be regarded as reactors that transform the input of various substances via precipitation, deposition, or human activities as they pass through soils and aquifers towards receiving streams. Understanding and modeling the variability of solute concentration in catchment waters require the identification of prevailing processes, determining their respective contributions to the observed transformation of substances, their interplay with hydrological processes, and the determination of anthropogenic impacts. However, numerous biogeochemical processes often interact in a highly non-linear way and vary on temporal and spatial scales, resulting in temporally and spatially varying water chemistry in catchments. This is particularly true for riparian wetlands. Processes in this catchment area often superimpose the influence of the hill slope (and largest) area of the catchment on surface water quality. Accordingly, the first part of this thesis (Study 1 and 2), focuses on the temporal and spatial variability of biogeochemical processes at the catchment scale. Therefore, the first aim was to identify the prevailing biogeochemical processes which affect the quality of catchment waters in two forested granitic catchments. Based on these results, (i) the long-term behavior of these processes was determined (Study 1) and (ii) hot spots of these processes at the catchment scale along different flow paths were identified (Study 2). The second part (Study 3) focuses on the interplay between hydrological and biogeochemical processes in a riparian wetland, with the aim of systematically tracing back the temporal patterns of stream water chemistry to different biogeochemical processes and antecedent hydrological boundary conditions in the wetland. The third part (Study 4 and 5) focuses on weathering processes with the goal (i) of identifying the mineralogical sources of the groundwater’s buffer capacity against acid atmospheric deposition in a forested granitic catchment and (ii) determining the mineralogical sources of the high cation loads in surface water, induced by intensive agricultural activities in two agricultural granitic catchments. To reach these aims, multivariate statistical methods of dimensionality reduction (linear Principal Component Analysis, non-linear Isometric Feature Mapping), a low-pass filtering of time-series, a Cluster analysis, and major and trace element ratios and strontium isotopes were used. A small number of biogeochemical process bundles explained 94% and 89% of the variance of the data set in Study 1 and 2, respectively. In Study 1, redox and topsoil processes, road salt and sulfate contamination were identified as predominating processes influencing water chemistry in the respective catchments. Low-pass filtered time series of component scores revealed a different long-term behavior at different sampling sites in both catchments, which could be traced back to the fraction of wetland area in the respective subcatchments as well as by the varying thickness of the regolith. Study 2 revealed that the upper 1 m topsoil layer could be considered as a biogeochemical hot spot for redox processes, acid-induced podsolization, and weathering processes along different flow paths. Up to 97% of the biogeochemical transformation of the chemical composition of soil solution, groundwater and stream water in the Lehstenbach catchment was restricted to this soil layer representing less than 2% of the catchment’s regolith. Wetland stream water, mobilized in the topsoil layer being considered a biogeochemical hot spot, showed a highly dynamic temporal pattern of component scores. Study 3 revealed four different types of wetland stream water chemical status, depending on the interplay between discharge dynamics, biological activity, and the water table position in the wetland. The sequence of different stream water types roughly followed a seasonal pattern, albeit being heavily modified by the respective hydrological boundary conditions for different years. Extended periods of low groundwater level in the second half of the growing season drastically changed the chemical boundary conditions, becoming evident in a drastic reoxidation of reduced species like sulfides and corresponding effects. Weathering processes are one of the predominating biogeochemical process bundles influencing water chemistry in forested catchments. Study 4 showed that the mineralogical sources of the groundwater’s buffer capacity against acid atmospheric deposition were dominated by the release of base cations from apatite dissolution, preferential cation release from feldspars and biotite, and feldspars weathering. In Study 5, determining the mineralogical sources of the high cation loads in surface water induced by intensive agricultural activities revealed a dominant manure contribution in the topsoil, and enhanced mineral dissolution (plagioclase and biotite) by fertilizer application in subsoils, becoming the unique source of base cations in the saprolite. Stream water chemistry differed from that of soil water, suggesting that stream water chemistry was dominated by elements issued from enhanced mineral and rock weathering. Soil acidification induced by agriculture allows the mobilization of cations stored in soil layers, enhances the rock weathering and accelerates plagioclase dissolution, which can highly influence stream water quality. Numerous biogeochemical, hydrological, and anthropogenic processes were found to interact with each other, mostly with non-linear patterns, influencing catchment water chemistry. The integral approach used in this thesis would be a useful prerequisite to develop accurate and parsimonious models commonly used for water management purposes by distinguishing between short- term and long-term shifts, reducing the number of processes to the predominating ones ultimately to be included in the model, focusing on hot spots and including spatial patterns where necessary and appropriate.
... Over the last 30 years numerous attempts have been undertaken to determine the emplacement ages and to refine the intrusion sequence of the late-Variscan granitoids in the northern Oberpfalz, SE Germany (e.g.Köhler et al. 1974;Köhler and Müller-Sohnius 1976;Wendt et al. 1986Wendt et al. , 1988Holl et al. 1989;Wendt et al. 1992Wendt et al. , 1994Siebel 1995aSiebel , 1995b). From existing data it was concluded that magmatism took place in two major pulses at ~325 Ma and between 315–310 Ma (Siebel et al. 1997). ...
... 4. Isolated outcrops, interpreted as dikes or sills in the region between Tirschenreuth and Mähring. Accounts on petrography, geochemistry and field relations of the granites sensu stricto have been given elsewhere (Voll 1960;Fischer 1965;Köhler et al. 1974;Madel 1975;Wendt et al. 1986;Richter and Stettner 1987;Wendt et al. 1988Wendt et al. , 1992Wendt et al. , 1994Siebel 1995aSiebel , 1995bSiebel et al. 1997). Only a brief description is given here. ...
Pb-evaporation zircon analyses coupled with a detailed cathodoluminescence (CL) study on the complete series of granitoids from the northern Oberpfalz, NE Bavaria, provide new evidence for the commencement and timing of late-Variscan magmatism. All granitoids analysed in this study were dated before by Rb-Sr and/or K-Ar methods. Investigated samples comprise medium-grained, I-type dioritic rocks (redwitzites), I/S-type granites (Leuchtenberg, Marktredwitz (G1), Zainhammer) and S-type granites (Falkenberg, Liebenstein, Mitterteich, Friedenfels, Steinwald, Flossenbrg, Brnau). The zircon evaporation technique reveals three groups of 207Pb/206Pb ages which are interpreted to represent magmatic crystallisation: (1) ages of 324-321 Ma are found in all analysed redwitzites and in almost all I/S-type granites; (2) the granites of Falkenberg and Liebenstein yield ages of ~315 Ma; (3) ages between 312 and 310 Ma are recorded in the Mitterteich, Friedenfels, Steinwald and Flossenbrg granites. Titanite crystals from different redwitzite bodies yield conventional U-Pb ages of 325-322 Ma, identical to the Pb-evaporation zircon data of these rocks. The S-type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages, indicating that some of the cores must have lost their radiogenic Pb. From the geochronological data, we infer that metamorphic conditions of the Variscan crust produced different granite types at different times. The data support a model involving an early period of mantle-related magmatism which postdates the final convergence stage of the Variscan orogen. This magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis, which is probably related to post-collisional extension of the thickened Variscan crust.
... There is abundant literature on tectonic settings of the granitoids, their ages, and the timing of metamorphism in the Variscan belt of Europe [33][34][35][37][38][39][40][41][42][43][44][45][46][47]. Magmatic activity there [25,35,40,[48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63] occurred over a time interval (320 to 290 Ma) which is similar to those in the Strandja massif. Taking into consideration S-type of intrusions the majority of researches assigned them to post-collisional tectonic settings. ...
The Strandja massif consists of metamorphic basement intruded by large Early Permian plutons of the Kirklareli type and overlain by Triassic metasedimentary cover. Together with its continuation in Bulgaria this massif forms an important link between the Pontides and the orogenic belts of Europe. Various types of orthogneisses constitute a significant part of the metamorphic basement however these rocks have until now escaped a particular study and therefore the Paleozoic history of the massif is essentially unknown. In this study these rocks are classified and mapped as hornblende-biotite, biotite-muscovite, and leucocratic orthogneisses. Their modal compositions correspond to quartz diorite, tonalite, granodiorite and trondhjemite. Geochemical data suggest a calc-alkaline trend of differentiation and metaluminous character of the parent magmas. Isotopic dating using the single zircon evaporation method has shown that magmatic ages of these orthogneisses cluster within a short time interval between 312±2 and 315 ±5 Ma in the Carboniferous. At the same time inherited ages of magmatic zircons in these rocks record a long lived magmatic activity between 340 and 650 Ma. We infer that the Carboniferous orthogneisses were formed in a magmatic arc that evolved atop of a mature continental basement. Previously established ([1, 2]) Early Permian magmatic event has been confirmed by additional age determinations constraining it at 257±6 Ma. Tectonic setting of this episode is also interpreted as subduction related taking into consideration its geochemical features and relationships with surrounding tectonic units.
... The second sub-group (3.2) includes highly potassic, weakly peraluminous, sometimes metaluminous, K-feldspar megacrystic biotite-granitoids. Important examples are the Weinsberg granite in the Southern Bohemian Batholith (Frasl and Finger, 1991;Finger and Clemens, 1995), the Leuchtenberg granite in the Oberpfalz (Siebel, 1995a), the Kristallgranit I in Bavaria (Kraus, 1962), the Oberkirch granite in the Schwarzwald (Emmermann, 1977) and the Knorrkogeland Hochweif3enfeldgneiss in the Hohe Tauern (Schermaier, 1993). These widespread megacrystic Kfs granitoids have a considerable silica range from felsic (~ 70 wt% SiO2) to more mafic quartz-monzonite end-members with silica contents of around 60 wt%. ...
... The second sub-group (3.2) includes highly potassic, weakly peraluminous, sometimes metaluminous, K-feldspar megacrystic biotite-granitoids. Important examples are the Weinsberg granite in the Southern Bohemian Batholith (Frasl and Finger, 1991;Finger and Clemens, 1995), the Leuchtenberg granite in the Oberpfalz (Siebel, 1995a), the Kristallgranit I in Bavaria (Kraus, 1962), the Oberkirch granite in the Schwarzwald (Emmermann, 1977) and the Knorrkogeland Hochweif3enfeldgneiss in the Hohe Tauern (Schermaier, 1993). These widespread megacrystic Kfs granitoids have a considerable silica range from felsic (~ 70 wt% SiO2) to more mafic quartz-monzonite end-members with silica contents of around 60 wt%. ...
During the Variscan orogenic cycle, central Europe was intruded by numerous granitoid plutons. Typological and age relationships show that the characteristics of the granitoid magmatism changed during the course of the Variscan orogeny. Five genetic groups of granitoids may be distinguished:1.
Late Devonian to early Carboniferous “Cordilleras” I-type granitoids (ca. 370-340 Ma): These early Variscan granitoids are mainly tonalites and granodiorites. They often have hornblende and occur in association with diorites and gabbros. They form plutonic massifs in the Saxothuringian unit, in Central Bohemia and the intra-Alpine Variscides. In terms of existing models, they can be interpreted as volcanic arc granites, being related to the subduction of early Variscan oceans. Models involving mantle sources and AFC may be feasible.
2.
Early Carboniferous, deformed S-type granite/migmatite associations (ca. 340 Ma): These occur in the footwall of a thick thrust in Southern Bohemia (Gföhl nappe) and seem to represent a phase of water-present, syn-collisional crustal melting related to nappe stacking.
3.
Late Visean and early Namurian S-type and high-K, I-type granitoids (ca. 340-310 Ma): These granitoids are mainly granitic in composition and particularly abundant along the central axis of the orogen (Moldanubian unit). This zone experienced a high heat flow at this time, probably as a consequence of post-collisional extension and magmatic underplating. Most of group 3 granitoids formed through high-T fluid-absent melting in the lower crust. Enriched mantle melts interacted with some crustal magmas on a local scale to form durbachites. Partial melting events in the middle crust produced a number of high-T/low-P, S- and I-type diatexites and some S-type granite magmas.
4.
Post-collisional, epizonal I-type granodiorites and tonalites (ca. 310-290 Ma): These plutons can be found throughout the Central European Variscides. However, most of them occur in the Alps (near the southern flank of the orogen). Such late I-type plutons could be related to renewed subduction along the southern fold belt flank, and/or to extensional decompression melting near the crust/mantle boundary. Post-collisional mantle or slab melting may have occurred in connection with remnant subduction zones below the orogen undergoing thermal relaxation and dehydration.
5.
Late Carboniferous to Permian leucogranites (ca. 300-250 Ma): Many of these rocks are similar to sub-alkaline A-type granites. Potential sources for this final stage of plutonism could have been melt-depleted lower crust or lithospheric mantle.
The Carboniferous Period is closely linked with the Variscan Orogeny which dominated many aspects of Carbonifeous basin development. In addition, tectonic, magmatic and metamorphic processes were closely linked with the ongoing orogenic activity (Figs 9.89 & 9.90). Compared with other orogens, the Variscan Orogeny had an unusally complex evolution and many aspects are the subject of active debate (Franke 2000). However, its development and architecture have become much clearer since the identification and correlation of plate tectonic elements (terranes, sutures, ophiolites, magmatic arcs, foreland fold-and-thrust belts and foreland basins), and the integration of palaeomagnetic and geophysical data with high-precision dating (Timmerman 2004). In general terms, the Variscan Orogeny may be considered the result of Devonian-early Carboniferous accretion onto the southern margin of Laurussia of various Gondwanaderived terranes or microplates (often composite) of predominantly Neoproterozoic (Cadomian/pan-African) crust, together with their Neoproterozoic-Silurian passive margin successions and accreted Ordovician-Devonian island arcs (Franke 1989; Ziegler 1990; Matte 1991). A number of distinct zones have been recognized across Variscan Europe, each of which shows a different sedimentary, magmatic and tectonomagmatic history (Figs 9.89 & 9.90). These zones can be defined as palaeogeographically coherent units that broadly retained their material integrity throughout the Variscan Orogeny.
Large-scale granitoid intrusion during the Variscan orogeny (370 to 250 Ma) in Europe above subduction zones of that time is assumed to be the result of a heat pulse due to slab detachment. This would show from a lateral trend in age of emplacement of all granitic bodies, migrating through the complete Variscan fold belt, and thus, a younging direction perpendicular to the former subduction zones. To test this, an inventory of intrusion ages (on the basis of U-Pb, Rb-Sr and K-Ar analysis), typology (I-or S-type) and location of 70 granitic bodies in the European Variscan fold belt, from the Armorican Massif to the Bohemian Massif, was made. From the collected data, no lateral trend in age is observed. However, a trend in age parallel to former subduction zones is shown in most massifs, with a younging direction towards the thrusting vergence. This is probably the result of nappe stacking. In the Armorican Massif and Massif Central, granites are all S-type, which is probably the cause of continental subduction. In central Europe, the typology is mixed S-and I-type, which can be caused by subduction of either continental or oceanic lithosphere.
The crustal unit penetrated during the German Continental Deep Drilling Program (KTB), the Zone of Erbendorf-Vohenstrauss (ZEV), comprises an association of metabasites and paragneisses and orthogneisses that underwent high- and medium-pressure metamorphic cycles during the early Palaeozoic. In this summary of the structural, petrological, geochemical and geochronological information from borehole and surface rocks, we show that geological models proposed prior to drilling have had to be significantly modified. Features of the ZEV, such as the dominant NW-SE trending structures, Devonian (>370Ma) medium-pressure, amphibolite facies, metamorphism (0.6-0.8 GPa, ~720°C), and earlier eclogite stage, are directly comparable with those of the nearby Bohemicum unit in western Bohemia. Intervening units, in contrast, exhibit NE-SW trending structures and Carboniferous (315-325 Ma), low-pressure, metamorphism: all units are cut by predominantly posttectonic granites (mostly
SmNd isotopic results are presented for late-and post-tectonic granitoids, comprising peraluminous granites and subordinate metaluminous dioritic rocks (redwitzites) from the Fichtelgebirge (FG) and Northern Oberpfalz (NOP), NE Bavaria. The data, combined with a number of earlier geochromological studies on these granitoids, place severe constraints on the crustal evolution of this region during the Carboniferous and pre-Carboniferous epoch.Redwitzites range in ϵNd(T) from −4 to 0 (inclusive data from P.K. Holl and coworkers). The ϵNd(T)-values of the granites s.s. are restricted to overlapping ranges of −8 to −3 (FG) and −8 to −2 (NOP). In both domains, the older granites (330-325 Ma) are characterized by ϵNd(T)-values of > −4 whilst the younger granites (315-305 and ∼290 Ma) have ϵNd(T)-values of < −4. The diversity observed in Nd isotopic characteristic is interpreted in terms of different source material. The Nd isotopic compositions of the redwitzites exhibit source heterogeneity suggesting mixing between mantle magma and crust. The Nd isotopic features of the older granites are consistent with the magmatic precursors having been generated by partial melting of pre-existing mature crust variably contaminated by mantle material or, alternatively, by melting of chemically less evolved crust resembling paragneisses of teh ZEV structural unit. The origin of the younger granites can be confidently linked to anatexis of common Moldanubian and Saxothuringian metasediments. Two-stage Nd model ages (TDM) of the granitoids are in the range 1.1–1.7 Ga. These ages provide further support of substantial involvement of pre-Phanerozoic crust in the generation of the granitoids.The Leuchtenberg granite and the G2 and G3 granites yield late Variscan SmNd isochron ages which are concordant with previous RbSr data on these granites. By contrast, the Nd isotopic data for Bärnau and Flossenbürg define pseudo-isochrons suggestive of mixing between contrasting felsic melts.
Wang et al. (2000) presented interesting new data on the occurrence of Hf-rich zircon in peralkaline granitic rocks. It is shown that the knowledge of the Hf distribution in zircon is important for the understanding of Zr/Hf ratios defined by whole-rock analysis. However, the processes leading to the unusual Zr/Hf fractionation are still poorly constrained. Some aspects are discussed further here. Analysis of available data demonstrates that extremely low Zr/Hf ratios found in topaz- and rare-metal bearing granites may be either a primary signature of some evolved granitic melts or, alternatively, they are produced during albitization of the rocks. Topaz- and rare-metal bearing granites may be P-rich and contain zircon moderately enriched in Hf (2 - 9 wt.% HfO2). Another group is P-poor and the Hf content in accessory zircon is high (up to 35 wt.% HfO2). Both types of intrusions may occur within a single orogen as demonstrated by the example of the Hercynian magmatism in the Erzgebirge/Krušné hory region (Germany/Czech Republic).
Late-Variscan granitoid plutons in western Bohemia (Bor, Waidhaus-Rozvadov) have distinct petrographic, geochemical and isotopic features that suggest different magmatic evolutions. The Bor pluton comprises a suite of metaluminous tonalites and quartz diorites (Bor I), weakly peraluminous (monzo-)granites and granodiorites (Bor II) and medium-aluminous, late vein-forming leucomonzogranites (Bor III). The Waidhaus-Rozvadov pluton is strongly peraluminous, comprising a cordierite-biotite granitoid (CBG), the Rozvadov granite (ROG), the Bärnau granite (BÄG) and the subordinate, highly evolved Kreuzstein (Křížový kámen) granite (KG). Geochemical parameters and initial87Sr/86Sr ratios straddle the boundary between I- and S-type granites in the Bor pluton and are characteristic of purely S-type granites in the Waidhaus-Rozvadov pluton.
The Bor II granitoids have been dated by the Rb-Sr whole-rock method at 341±17 Ma (ISr = 0.70724±0.00060). K-Ar biotite and muscovite ages of all units of the Bor pluton are mainly in the range 321-315 Ma. The K-Ar mineral ages are in good agreement with recently published U-Pb zircon data of these rocks. The different units of the Waidhaus-Rozvadov pluton have yielded less well-constrained Rb-Sr whole-rock ages, ranging from 313 to 300 Ma. However, the intrusion sequence is constrained by K-Ar muscovite ages (312-302 Ma), which define a systematic decrease towards the chemically more evolved granite types. Taken as a whole, it seems likely that the new radiometric ages characterize two temporally distinct periods of late-Variscan granitoid intrusion. The regional significance of these periods is emphasized by contemporaneous ages previously found in the adjacent northeastern Bavarian granitoids.
The initial Sr and Nd isotope systematics indicate that the Bor and the WaidhausRozvadov plutons were derived from different source rocks. The Bor granitoids reflect the influence of less evolved crustal material which may have been similar to paragneisses of the Teplá-Barrandian region, including the Zone of ErbendorfVohenstrauß (ZEV). The Waidhaus-Rozvadov granitoids probably resulted from anatexis of rocks resembling surrounding Moldanubian paragneisses or metapelites. In addition, the two plutons exhibit poorly defined, opposite trends of εNd(T) variation which are ascribed to assimilation processes.
Recent research on ultrahigh-pressure rocks in Val Gilba and the Brossasco area has revealed the complete textural preservation of large volumes of a premetamorphic granite, together with its former contact aureole, that was subducted to depths of > 100 km, as indicated by coesite pseudomorphs. Petrographic studies indicate that, although most minerals have been affected by metamorphism, biotite and potassium feldspars in the rock are largely preserved except for thin peripheral rimming of biotite by phengite and garnet. RbSr analyses on seven biotite samples fail to produce a valid isochron, indicating either incomplete resetting, or inheritance of radiogenic Sr, during metamorphism. Ar/Ar data from two biotites likewise yield very irregular patterns that are consistent with an episode of severe Ar loss from a pre-Alpine parent rock, followed later by addition of inherited radiogenic Ar. The biotite RbSr and KAr systems have therefore been extensively disturbed during Alpine metamorphism in spite of the largely unaltered appearance of the crystals in thin sections. Attempts to measure UPb ages with apatite also failed. Although the geochronological studies yielded no reliable ages, Pb and Sr ratios from four potassium feldspar samples are within the ranges found in known Hercynian rocks, providing new evidence that Hercynian rocks played a prominent role as source materials for the Dora Maira rocks. Sr ratios from two apatite samples agree closely with the potassium feldspar values. The ratios are also substantially lower than those found in the whiteschists in completely recrystallized rocks at Parigi, ca. 3 km north of the present rocks. These results indicate that the undeformed rocks in Val Gilba-Brossasco area are less disturbed by Alpine ultrahigh-pressure metamorphism, thereby preserving the record of the protoliths somewhat better than do the completely recrystallized rocks at Parigi to the north. The Pb and Sr isotope data further show that the minerals of the undeformed rocks failed to reach isotopic equilibrium despite subduction to > 100 km and heating to > 700°C.
Bimodal metavolcanic rocks, granitic gneisses and metasediments are associated in the Frankenberg massif, Germany. These rocks are faulted against underlying very low-grade Palaeozoic sequences and adjacent metamorphic complexes of the Variscan basement. The granitic gneisses record an Rb–Sr whole-rock isochron age of 461±20 Ma that is taken as at least a minimum protolith age. The bimodal meta-igneous suites are interpreted to have formed during rifting of the Gondwana continental margin in the Cambro-Ordovician. The various metamorphic units have all experienced a common P–T history. The peak-pressure stage is constrained to around 490–520 °C and 10–14 kbar (10–12 kbar being most realistic). The metamorphism proceeded along a clockwise P–T path towards conditions of around 580–610 °C and 7–8.5 kbar at the thermal peak followed by a final low-pressure overprint which spanned amphibolite facies to prehnite–actinolite facies temperatures. Owing to a secondary Rb–Sr whole-rock isochron age of 381±24 Ma, interpreted to date the retrograde stage, the whole metamorphic cycle in the Frankenberg massif is ascribed to the late Silurian–early Devonian high-pressure event widely recorded in the European Variscides. The antiformal complexes bordering the Frankenberg massif underwent a well-documented early Carboniferous metamorphism, suggesting that the Frankenberg massif constitutes a klippe which was overthrust towards the end of this second metamorphic cycle.
The role of different minerals in base cation release and thus the increase of buffering capacity of groundwater against acid deposition is controversially discussed in the literature. The ⁸⁷Sr/⁸⁶Sr ratios and base cation concentration were investigated in whole rock leachates, mineral separates, precipitation, soil solution, groundwater and stream water samples in the Lehstenbach catchment (Germany) to identify the weathering sequence of the granite bedrock. Three different approaches were followed in parallel. It was assumed that the contribution of different minerals to base cation supply of the groundwater with increasing weathering intensity would be observed by investigating (1) unweathered rock leachates, deep groundwater and shallow groundwater, (2) groundwater samples from new groundwater wells, reflecting the initial weathering of the drilled bedrock, and groundwater from wells that were drilled in 1988, (3) stream water during baseflow, dominated by deep groundwater, and stream water during high flow, being predominantly shallow groundwater. Whereas the first approach yielded consistent patterns, there was some evidence that groundwater from the new wells initially reflected contamination by the filter gravel rather than cation release in an initial stage of weathering. Time series samples of stream water and groundwater solute concentrations and isotope ratios turned out to reflect varying fractions of soil water and precipitation water at baseflow and high flow conditions rather than varying contributions of different minerals that prevail at different stages of granite weathering.
The Variscan Hauzenberg pluton consists of granite and granodiorite that intruded late-to postkinematically into HT-metamorphic rocks of the Moldanubian unit at the southwestern margin of the Bohemian Massif (Passauer Wald). U–Pb dating of zircon single-grains and monazite fractions, separated from medium-to coarse-grained biotite-muscovite granite (Hauzenberg granite II), yielded concordant ages of 320 ± 3 and 329 ± 7 Ma, interpreted as emplacement age. Zircons extracted from the younger Hauzenberg granodiorite yielded a 207 Pb– 206 Pb mean age of 318.6 ± 4.1 Ma. The Hauzenberg granite I has not been dated. The pressure during solidification of the Hauzenberg granite II was estimated at 4.6 ± 0.6 kbar using phengite barometry on magmatic muscovite, corresponding to an emplacement depth of 16-18 km. The new data are compatible with pre-existing cooling ages of biotite and muscovite which indicate the Hauzenberg pluton to have cooled below T = 250–400 °C in Upper Carboniferous times. A compilation of age data from magmatic and metamorphic rocks of the western margin of the Bohemian Massif suggests a west-to northwestward shift of magmatism and HT/LP metamorphism with time. Both processes started at N 325 Ma within the South Bohemian Pluton and magmatism ceased at ca. 310 Ma in the Bavarian Oberpfalz. The slight different timing of HT metamorphism in northern Austria and the Bavarian Forest is interpreted as being the result of partial delamination of mantle lithosphere or removal of the thermal boundary layer.
Geochemical and Nd-Sr isotopic compositions and U-Pb zircon ages of two Variscan granites (Neunburg and Oberviechtach) from southern Oberpfalz, NW Bohemian massif, have been investigated in order to place constraints on their formation and on the crustal reworking. Both granites exhibit similar mineralogical, chemical and isotopic characteristics. They have peraluminous compositions (A/CNK ratios 1.2-1.3) and display high K2O/Na2O ratios of 2.2-2.3, consistent with typical S-type granites. In terms of trace elements, they show an enrichment of LREE and strong fractionation between LREE and HREE (LaN/YbN ratios 46 to 60). Compared with the primordial mantle, distinct negative anomalies of several trace elements (Ba, Sr, Nb and Ti) are also observed in both granites. They are further characterised by low initial kNd-values of ш.2 to ъ.2 and high initial 87Sr/86Sr ratios of 0.7114 to 0.7147. Zircon U-Pb data indicate that the intrusion of both granites shortly post-dates the HT-LP metamorphism of the Moldanubian basement and crystallised at about 320 Ma. The samples studied contain zircons mostly having xenocrystic cores with diverse morphologies. These inherited zircons have Early Proterozoic to Early Palaeozoic ages. This points to melting of sources comprising substantial sedimentary rocks. The LaN/YbN and TbN/YbN ratios of both granites are the highest so far reported from granitoids within this region. Melting of lower crustal rocks leaving garnet as a restite phase in the source provides a viable mechanism to reproduce the REE characteristics.
The Leuchtenberg granite (Oberpfalz, NE Bavaria) displays a continuous differentiation trend ranging from mildy peraluminous, coarse-grained, porphyritic biotite granites (BG) to strongly peraluminous, medium- to fine-grained, garnet-bearing muscovite granites (GMG). The Rb–Sr and K–Ar age determinations of whole-rock and mineral samples from the granite and associated intermediate rocks (redwitzites) have revealed two divergent age gradients: Rb–Sr wholerock dates decrease and initial 87Sr/86Sr ratios increase for successively more evolved subsets of the granite. All BG samples (87Rb/86Sr=2–16) yield a date of 3262 Ma with a low initial 87Sr/86Sr ratio of 0.707780.00013 (1), while all GMG samples (87Rb/86Sr=70 to 1000) yield a younger date of 3172 Ma with an enhanced initial 87Sr/86Sr ratio of 0.71460.0039. The K–Ar measurements on biotites and muscovites give closely concordant dates for the GMG (326–323 Ma) and the southern lobe of the BG (324–320 Ma). The northern lobe of the BG, including the redwitzites, shows a well-defined trend of decreasing K–Ar dates from 320 Ma to 300 Ma towards the northwest. Critical consideration of both isotope systems leads to the conclusion that the Rb–Sr system of the GMG was disturbed by a later hydrothermal event. The ca. 326 Ma whole-rock Rb–Sr date for the BG is not in conflict with any of the K–Ar mineral dates and is taken as approaching the crystallization age of the Leuchtenberg granite. The K–Ar age progression within the northern lobe of the BG indicates that this part either cooled down over a protracted period of some 20 Ma or experienced reheating at ca. 300 Ma. The study highlights the potential of combined Rb–Sr and K–Ar dating in deciphering detailed chronology on the scale of a single igneous intrusion.
The Rozvadov Pluton is a complex of mainly Variscan granitoid rocks situated near the Bohemian-Bavarian border between Brnau, Tachov, Rozvadov and Waidhaus, 25 km ESE of the KTB site. Five mappable units can be distinguished, which intruded as folows: (1) slightly deformed leucocratic meta-aplite/metapegmatite dykes with garnet and tourmaline; (2) a complex of cordierite-bearing granitoids, which have been divided into three facies (a) biotite granite with cordierite (at the margin of the complex), (b) biotite-cordierite granite and (c) cordierite tonalite (in the centre of the complex; (3) fine-grained biotite granite of the Rozvadov type with associated pegmatite bodies; (4) two-mica Brnau granite; and (5) geochemically specialized albite-zinnwaldite-topaz granite (Krov kmen/Kreuzstein granite) with indications of Sn-Nb-Ta mineralization and associated phosphorus-rich pegmatite cupolas. Rare earth element data suggest that meta-aplite/pegmatite dykes are the result of a batch partial melting process, whereas the compositional variation of the other rock types was mainly controlled by fractional crystallization. The genesis of the cordierite granitoid suite is best explained in terms of a batch melting of metapelitic source followed by crystallization of a cordierite-rich cumulate and K-feldspar enriched melt. The leucocratic pluton constituents — the meta-aplites and the Brnau and Kov kmen granites are rich in phosphorus (0.5–0.8%). The main carriers of phosphorus are alkali feldspars, especially K-feldspar (up to 0.8% P2O5). The presence of P-rich leucocratic granites is one of the features distinguishing the Variscan granitoids within the Moldanubian zone from the nearly contemporaneous granitoids in the Saxothuringian zone.
The late Variscan granitoids of the NW Bohemian massif (northeast Bavaria, west Bohemia) constitute four partly contiguous granitoid complexes: Fichtelgebirge, northern Oberpfalz, Waidhaus-Rozvadov and Bor, incorporating more than 20 intrusive units. Based on gravity data, the granites can be modeled 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, composition ranges from less evolved dioritic rocks, known as the redwitzite suite, to highly evolved granites. 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 ɛNd(T) values (1 to –4). Sr–Nd isotopes of the redwitzites show contamination trends towards the granites suggesting mixing between mantle magma and crustal granitic melts. An older plutonic association (granites of Bor, Leuchtenberg, 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 ɛNd(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 Teplá Barrandian. It is postulated that the older granites were formed either by partial melting of ZEV or Teplá Barrandian crust, or alternatively, of preexisting mature 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 ɛNd(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 complex and (c) a third phase with emplacement ages of 305–295 Ma restricted to the Fichtelgebirge.
Orthogneisses and acid metavolcanic rocks from the Fichtelgebirge, NE Bavaria, Germany, are predominantly chemically evolved (monzo)granites and rhyodacites–dacites, respectively. The metavolcanic rocks are variably tectonized and include samples with anomalously low CaO, Na 2 O and MnO (<0.01 wt %) and high SiO 2 (>80 wt %) and K 2 O (>6 wt %) concentrations, implying selective element mobility during post-eruptive events. Sm–Nd isotope data for samples from three main orthogneiss units (Wunsiedel, Selb, Waldershof) yield a composite Sm–Nd whole-rock isochron corresponding to an age of 560 ± 45 Ma. This age estimate is constrained, however, to be less than ∼560 Ma by the presence of detrital Cadomian zircons in the country rocks. Fifteen specimens from the Wunsiedel orthogneiss give an Rb–Sr whole-rock isochron of 480 ± 4 Ma with an initial 87Sr/86Sr ratio of 0.7095 ± 0.0007 (MSWD = 2.7). Rb–Sr isotope data from the Waldershof orthogneiss and the metavolcanic rocks suggest, however, that, in general, Sr isotopic equilibrium, if ever reached, was significantly modified during later events. Taking recent geochronological literature data into account, it now appears that Early Palaeozoic acid magmatism in the Fichtelgebirge commenced with the intrusion of the orthogneiss precursors during the Early and Mid Ordovician and ended with the eruption of the volcanic successions during the Late Ordovician. K–Ar ages of the investigated rocks range from 316 to 298 Ma for muscovites and from 306 to 280 Ma for biotites, demonstrating thermal influences from Late Visean to Late Stephanian (325–290 Ma) granite intrusions. The involvement of dominantly crustal-derived melts is considered to account for the peraluminous character (A/CNK > 1.08), high initial 87Sr/86Sr (> 0.709) and negative ϵ Nd(500 Ma) (−2.9 to −6.4) of the intrusive and volcanic rocks. The generation of the melts is assumed to have taken place within an overall extensional geodynamic setting. The wider range in ϵ Nd(500 Ma) for the metavolcanic rocks (−3.8 to −6.4; orthogneisses: −2.9 to −4.0) suggests that differential admixture of a pelagic sedimentary component during emplacement may have occurred. Nd model ages range from 1.5 to 1.7 Ga and are consistent with those of granitoids from the Erzgebirge and the Lausitz but completely different from those of metavolcanic rocks from the Thüringer Wald. In addition, Nd model ages of the Early Palaeozoic granitoids in the Fichtelgebirge are consistent with those of the Late Carboniferous granitoids from the same area. This suggests similar source material for the pre-Variscan and the late Variscan Fichtelgebirge granitoids.
The Strzegom-Sobotka granitic massif (Sudetes, SW Poland) consists of four
main rock types: hornblende- biotite monzogranite and biotite monzogranite (in the
west); two-mica monzogranite and biotite granodiorite (in the centre), and biotite granodiorite
(in the east). The hornblende - biotite monzogranite, two-mica monzogranite
and biotite granodiorite exhibit different magmatic and post-magmatic features, suggesting
that they represent separate magma batches. The relationship of the biotite monzogranite
to the neighbouring varieties cannot be established petrographically.
According to the Rb - Sr isotope data the hornblende - biotite monzogranite, the
biotite granodiorite and the biotite monzogranite have the same age of about 280 Ma.
Their initial 87Sr/86Sr ratios differ markedly, however, at 0.7098 (Hb - Bi monzogranite),
0.7082 (Bi monzogranite) and 0.7058 (Bi granodiorite). The two-mica granite is
significantly older (ea. 330Ma), and has a low initial ratio (0.705).
These results emphasize the composite character of the Strzegom-Sobotka massif,
which consists of at least four different magmatic batches emplaced during two widely
separated phases of magmatic activity. There is no relationship between the petrographic
features, emplacement age and the derivation of the magmas as it may be inferred
from initial Sr isotope composition. Rather, the isotopic pattern appears to be controlled
by the geographic situation of each unit in the composite pluton, suggesting that the
Strzegom-Sobotka granites straddle the boundary between two contrasting crustal
blocks.
Average contents of major element oxides in granites and porphyries of the Krusne hory Mts. pluton were calculated from silicate analyses. A significant difference was found between the granites of the Older and Younger intrusive Complexes. Transitional granites were also characterized by geochemistry. The distribution maps of Na, K, Rb, F, Sn and Sr were prepared from sampling of granite and porphyries outcrops in the Western and Eastern partial plutons and compared with the results of petrological studies. The granitoids of the Krusne hory Mts.pluton are metallogenetically specialized. Their youngest types belong to granites highly enriched in volatile lithophile elements . They are extremely enriched in Li and F but are relatively poor in B, Nb and Ta. The results can be used in prospection for tin and tungsten ores in the area of the pluton.
The late Palaeozoic Moldanubian plutonic terrain of Upper Austria consists of granitoids that range from S-type to I-type in composition although many units show transitional compositional characteristics. The Austrian terrain displays some of the features of continent-margin “main-arc” batholiths: the expanded compositional range (monzodiorite to leucogranite) of the rock types, the high-Sr I-types and the low- to initials (0.705–0.708) of these I-types. However, there are also important differences: the Austrian I-types are low-CaO and high-K2O units, and transitional- and S-type granitoids are volumetrically dominant in the Austrian terrain. There is a scarcity of gabbros and an absence of rocks with positive-ϵNd initials. The Austrian terrain is dominated by granitoids of crustal or predominantly crustal origin. We suggest that these formed in an “inner-continental arc” setting rather than a main arc, and can be correlated with those of the Schwarzwald, F.R.G. Together, these are interpreted as components of a Moldanubian inner-continent plutonic belt.
A method is described whereby a normative composition can be derived for a volcanic or plutonic rock which, unlike most norms, is a close approximation to the modal composition. The normative composition is calculated in terms of Niggli-Barth cation equivalents using a method closely allied to the CIPW system. Two variants are available for volcanic rocks and three for plutonic rocks. The main feature of the method is a mathematical device for representing the three normative feldspar components as two normative feldspars. Normative feldspar components are calculated with reference to the 900o, 650o and 450oC sections of the phase diagram for the system albite- orthoclase-anorthite. The results obtained are discussed and compared with Rittmann AMS norms. A Fortran IV computer programme was specially written for these calculations. Apart from volcanic and plutonic norms, the programme provides Niggli values, ratios of selected elements and the CIPW norm (in wt.%) of the relevant igneous rock. All five variants of the volcanic and plutonic norms can be calculated in one operation.-P.Br.
A geothermometer based on the exchange of Mg-Tschermak's component between muscovite and biotite ([MgSiAl-2]B = [MgSiAl-2]M) is empirically calibrated using data from 43 rocks having the assemblage muscovite + biotite + quartz + plagioclase + garnet + Al2SiO5. Temperatures and pressures for the calibration are determined through the simultaneous application of garnet-biotite geothermometry and GASP geobarometry. The quality of fit is indicated by a multiple correlation coefficient of 0.92. A standard deviation of 22 K in the residuals may be easily explained through the propagation of errors associated with microprobe analysis. Use of the muscovite-biotite geothermometer should be restricted to micas that are compositionally similar to those in the calibration data set. -Author
The Variscan basement at the Continental Deep Drilling Site (KTB) Oberpfalz on the western margin of the Bohemian Massif is composed of three polyphase-deformed structural units: the Saxothuringian, the Moldanubian and MP to HP metamorphic nappe complexes of the Zone of Erbendorf-Vohenstrauß (ZEV) and the Münchberg Massif (MM). The boundary between the Saxothuringian und Moldanubian, which is interpreted as a cryptic suture, is represented by the northwestern rim of the HT-mylonite belt of the Zone of Tirschreuth-Mähring (ZTM). This forms part of a formerly active continental margin, whereas the Saxothuringian terrane represents a formerly passive continental margin. Deformation of the Moldanubian active continental margin already began during the oceanic subduction stage and was therefore a longer-lasting, more penetrative deformational event than in the Saxothuringian, where the whole Variscan deformation is related only to the collisional stage.
The klippen of the ZEV and MM form part of a previously coherent nappe complex which was originally connected with the western part of the Bohemian (Zone of Tepla-Taus, ZTT) and now covers the Saxothuringian-Moldanubian suture. The Erbendorf Line near Erbendorf is a nappe boundary. Its interpretation as suture zone can no longer be upheld.
The Variscan crustal structure evolved during multiphase deformation in which two main events can be distinguished. The older events up to D3 are characterized by NW-directed tectonic transport. This is revealed by the seismic crustal structure which is characterized by dominantly SE-dipping reflectors in all NW-SE-directed reflection seismic profiles. Based on reflection seismic data, D2-backfolding and backthrusting of the Saxothuringian seems to be restricted to higher crustal levels and is interpreted as having resulted from crustal wedging and ramp tectonics. In the roof of a crustal wedge foreland-dipping backthrusts and antivergent folds develop, whereas foreland-directed overthrusting continues inside and below the wedge.
The younger event is characterized by NW-trending and SW-vergent D4-folding and thrusting and merges into late-orogenic dextral wrench faulting and extensional tectonics. The ZEV nappe forms a D4-syncline, in which the rocks have been preserved from erosion. The D4-deformation, which increases in intensity to the south, documents a reorientation of the regional stress field at the southwestern rim of the Bohemian Massif. This is interpreted on the basis of geophysical and geological data as resulting from a late-collisional indentation process.
Variscan crustal convergence was probably much stronger than previously assumed. Initial; oceanic subduction zones have been completely overridden by continental crust. Therey, the complexly deformed cryptic sutures seen at the present outcrop level of the crystalline basement have lost their contact woth the previous subduction zone. During collision, terranes previously separated by oceanic crust were thus thrust over one another, so that suture zones widely separated before collision are now closely neighboring or oevrridden by other terranes. The latter is presumably the case at the northwestern margin of the Bohemian Massif, where the Bohemian terrane overthrust the Saxothuringian-Moldanubian suture zone.
Based upon various experimental data, 3 kbar (11 km) is commonly considered the minimum P at which primary plutonic muscovite can crystallize. Other evidence suggests that apparently primary muscovite has crystallized at <11 km, indicating an apparent conflict between experimental and field data. New data from 41 samples representing 16 plutons reveal that primary and secondary muscovites have similar composition, but primary ones are slightly richer in Ti, Na and Al and poorer in Mg and Si. Plutonic muscovite is far from the ideal muscovite composition, and the additional components may appreciably affect its stability.-S.A.K.
U–Pb zircon and Rb–Sr whole-rock analyses from various gneisses and plutonie rocks of the Moldanubian and Moravo-Silesian zones and the stable foreland of the Hercynian (Variscan) orogenic belt indicate that most of the crust in Central Europe was first formed during the Cadomian orogeny which straddles the Precambrian–Cambrian boundary. Zircons, however, have a memory of older ages which correspond with those of events known in Fennoscandia. The new radiometrie data are consistent with the stratigraphie record in that they do not provide any evidence for a major early Palaeozoic tectonothermal event between the Cadomian and Hercynian orogenies.
Granulites from two localities in the Moldanubian zone yield U–Pb zircon ages of 345 ± 5 Ma; discordant zircon data points indicate that the granulite facies metamorphism was not of long duration. Tectonic units containing these high grade rocks were emplaced amongst amphibolite facies rocks during an event of widespread shearing which has been dated at 341 ± 4 Ma on the basis of a lower U–Pb zircon intercept age from one of the sheared gneisses and 338 ± 3 Ma U–Pb ages from monazites. Rb–Sr muscovite ages of 331 ± 5 Ma from pegmatites axial planar to asymmetrical folds date the last stage of SE-directed simple shear. A Rb–Sr whole-rock isochron of 331 ± 4 Ma from a principal magmatic type of the Central Bohemian pluton confirms the field evidence that the large NE-trending plutons of the Moldanubian zone were emplaced during a late stage of the deformation. The strong disturbance of the U–Pb zircon isotopic system in the sheared gneisses suggests U loss while a high U/Th ratio in monazite from one of these tectonised rocks suggests the simultaneous passage of hydrothermal fluids. Thus a crustal source is indicated for the uranium deposits of the Moldanubian zone.
Critical to any plate tectonic model for the development of the Middle European Hercynides was the existence of an ocean in Early Devonian times which separated a North European continent from a South European continent(s). The northward movement of the South European continent over a shallowly-dipping subduction zone and subsequent continental collision can explain the high T–low P metamorphism and the imbricated tectonic style of the Moldanubian zone and adjacent Moravo-Silesian zone along the southeastern Hercynian foreland. The temporal separation of granulites and granites implies distinct conditions of formation and it has been suggested that the plutonism, following on from the imbrication of the Cadomian crust, was initiated by the subduction of wet oceanic sediments.
Contrary to simple generalizations about their behavior, light
rare-earth elements (LREE) do not act as incompatible elements in very
felsic magmas. In fact, LREE concentrations typically decrease, often
drastically, during differentiation of such magmas. The simplest
explanation for this depletion involves the separation of minute, easily
overlooked quantities of an extremely LREE-rich accessory mineral,
either monazite or allanite. Our data indicate that felsic liquids with
< 50 ppm LREE may be saturated in either of these accessories and
that the concentration required for saturation
decreases in increasingly felsic liquids. This
accounts for incompatible behavior of LREE even at high concentrations
in mafic magmas in contrast to compatible behavior at low concentrations
in felsic magmas. Partitioning of LREE into solid rather than liquid has
important implications for trace-element and Nd-isotope modeling of
crustal anatexis, as well as for magma differentiation.
SmNd isotopic results are presented for late-and post-tectonic granitoids, comprising peraluminous granites and subordinate metaluminous dioritic rocks (redwitzites) from the Fichtelgebirge (FG) and Northern Oberpfalz (NOP), NE Bavaria. The data, combined with a number of earlier geochromological studies on these granitoids, place severe constraints on the crustal evolution of this region during the Carboniferous and pre-Carboniferous epoch.Redwitzites range in ϵNd(T) from −4 to 0 (inclusive data from P.K. Holl and coworkers). The ϵNd(T)-values of the granites s.s. are restricted to overlapping ranges of −8 to −3 (FG) and −8 to −2 (NOP). In both domains, the older granites (330-325 Ma) are characterized by ϵNd(T)-values of > −4 whilst the younger granites (315-305 and ∼290 Ma) have ϵNd(T)-values of < −4. The diversity observed in Nd isotopic characteristic is interpreted in terms of different source material. The Nd isotopic compositions of the redwitzites exhibit source heterogeneity suggesting mixing between mantle magma and crust. The Nd isotopic features of the older granites are consistent with the magmatic precursors having been generated by partial melting of pre-existing mature crust variably contaminated by mantle material or, alternatively, by melting of chemically less evolved crust resembling paragneisses of teh ZEV structural unit. The origin of the younger granites can be confidently linked to anatexis of common Moldanubian and Saxothuringian metasediments. Two-stage Nd model ages (TDM) of the granitoids are in the range 1.1–1.7 Ga. These ages provide further support of substantial involvement of pre-Phanerozoic crust in the generation of the granitoids.The Leuchtenberg granite and the G2 and G3 granites yield late Variscan SmNd isochron ages which are concordant with previous RbSr data on these granites. By contrast, the Nd isotopic data for Bärnau and Flossenbürg define pseudo-isochrons suggestive of mixing between contrasting felsic melts.
Sixty-six mineral concentrates from the granitic Murrumbidgee Batholith, A.C.T., were analysed for major and trace elements to explore their chemical variation.Analyses of eighteen microcline samples show that Ba, Sr, Rb and Ga vary in a regular manner which can be correlated with the SiO2 content of their host rock, but Pb and Y vary irregularly; all six trace elements vary independently of the major element composition of the microcline.Twenty-two analyses show a high correlation between the anorthite content of plagioclase and the CaO and SiO2 contents of the host rocks. Ba and Sr are significantly correlated with the Ca content of the plagioclase, and therefore with the SiO2 content of the host rocks, but Pb, Rb and Y behave irregularly. Ga content of the plagioclase decreases with increasing SiO2 content of the host rock and shows a high correlation with Al content of the plagioclase.All of twenty-two biotites analysed are rich in Al and the Al content of both tetrahedral and octahedral sites appears to be related to host rock paragenesis and, therefore, to bulk rock composition. Fe2+, Mn, Li, Nb, Zn, Ga, Rb and Pb contents of the biotites increase with increasing SiO2 content of their host rocks, whilst Mg, V, Cr, Co and Na decrease; Cu, K and Ba show no correlation with host rock SiO2 content.Analyses of two muscovites indicate preferential enrichment in Nb and Ga.Chemical similarities between hornblendes occurring in xenoliths and in enveloping tonalite at an isolated locality in the batholith suggest a xenocrystal origin for the hornblende in the tonalite.Almandine-spessartine garnet occurs in two leucogranite intrusions and is probably a primary mineral phase precipitated from Mn-rich magma.Regular patterns discerned in the partition of trace elements between coexisting mineral pairs suggest that a close approach to equilibrium has been attained in the intrusions, but that three different types of intrusions recognised in the batholith have equilibrated at different temperatures.
Dioritic intrusives and granites from the northwestern Bohemian Massif, West Germany were analyzed for the chemical and isotopic composition of Rb-Sr and Sm-Nd. The Marktredwitz intrusive suite and the Tirschenreuth-Mähring sill intrusions yield apparent Rb-Sr whole-rock isochron ages of 468 ± 9 Ma and 470 ± 33 Ma, respectively, while the Reuth-Erbendorf granodiorites display a scatter with an apparent age of 545 ± 16 Ma.The Rb-Sr whole-rock ages conflict with geological evidence: the diorites intruded metamorphic country rocks which experienced an Early Devonian metamorphism. Contact relationships between the diorites and porphyritic granites indicate nearly contemporary emplacement. The intrusion ages of two porphyritic granites from this area have been dated at 319 Ma and 311 Ma.The diorites display a variation in ϵNd (320 Ma) ranging from −1.0 to −4.4, whereas the two granites yield homogeneous initial ϵNd values of −3.7 and −6.6. The variations in elemental concentrations and initial isotopic ratios clearly show a mixing trend within the dioritic rock suites. The mixing partners are a presumably mantle-derived magma and crustal components of varying composition.The isotope systematics of the diorites and the concordance of their Rb-Sr whole-rock age values with Caledonian rock-forming events in Central Europe point to a pre-intrusive Caledonian event inherited in these rocks. We suggest that the diorites were formed by multistage anatectic processes.
This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.
Samarium-neodymium isotope systematics provide a means of determining the age of the continental crust, where "age' refers to the amount of time the crustal rock material has been isolated from the convecting mantle. This age is referred to as the Sm-Nd model age or the mantle separation age. Methods are presented for treating isotopic data on continental rock materials to obtain meaningful mantle separation ages. The methods are applied to the Precambrian and Mesozoic rocks of the southwestern United States to produce a mass-age distribution for the region, which represents 1% of the global continental mass. The results suggest episodic crustal growth, with short growth periods at circa 2.8, 1.8, and 0.1 Ga. About 90% of the crust was formed by 1.8 Ga, the remaining 10% was added in the Phanerozoic. The mean age of this section of continent is determined to be 1.84 Ga. There is no correlation between crustal thickness and the crust formation age in the area studied; the age-area and age-mass curves are nearly identical. -from Authors
The biotites from a series of rocks ranging in composition from tonalite to granite have been analysed for both major and trace elements.The relations between chemical composition and paragenesis of the biotites are studied. Most biotites co-exist with potassium feldspar and ilmenite. Variations in composition can be correlated with the occurrence of amphibole, primary muscovite and aluminosilicates in the rocks.Variation diagrams of the trace element contents and element ratios of biotite are compared to those of the host rocks. Fractionation of elements can be defined more accurately as the influence of other mineral phases is eliminated.Variations in the proportions of the octahedrally co-ordinated Al, Ti and Fe3+ are correlated with the conditions of crystallization and comparisons made with biotites from other suites of calc-alkali rocks.In the light of the experimental data available, the petrographic observations and the chemical data it is apparent that biotites crystallized from systems in which fO2 was buffered, its values remaining close to that of the buffer FMQ. From the same data, a temperature of 800°C for fO2 = 10−14to 10−15 bars is deduced as prevalent during the crystallization of the tonalites while for the granites, at a temperature of crystallization of 680°C, fO2 = 10−16to 10−18 bars.A calc-alkali trend of fractionation is therefore apparent with decreasing fO2 while fH2O2 remains relatively high.
Following and extending the early work of Velde (1965) the pressure-temperature dependence of the compositions of potassic white micas coexisting with K-feldspar, quartz, and phlogopite in the model system K2O-MgO-Al2O3-SiO2-H2O was investigated up to fluid pressures of 24 kbar by synthesis experiments. There is a strong, almost linear increase of the Si content per formula unit (p.f.u.) of phengite, ideally KAl2−xMgx[Al1−xSi3+xO10] (OH)2 with pressure, as well as a moderate decrease of Si (or x) with temperature. The most siliceous phengite with Si near 3.8 p.f.u. becomes stable near 20 kbar depending on temperature. However, contrary to Velde's assumption, these phengites coexisting with the limiting assemblage are invariably not of an ideal dioctahedral composition (as given by the above formula) but have total octahedral occupancies as high as about 2.1 p.f.u.
The stability field of the critical assemblage phengite — K-feldspar — phlogopite — quartz ranges, in the presence of excess H2O, from at least 350° C to about 700° C but has an upper pressure limit in the range 16–22 kbar, when K-feldspar and phlogopite react to form phengite and a K, Mg-rich siliceous fluid.
For the purpose of using these phase relationships as a new geobarometer for natural rocks, the influence of other components in the phengite (F, Fe, Na) is evaluated on the basis of literature data. Water activities below unity shift the Si isopleths of phengite towards higher pressures and lower temperatures, but the effects are relatively small. Tests of the new geobarometer with published analytical and PT data on natural phengite-bearing rocks are handicapped by the paucity of reliable values, but also by the obvious lack of equilibration of phengite compositions in many rocks that show zonation of their phengites or even more than one generation of potassic white micas with different compositions. From natural phengites that do not coexist with the limiting assemblage studied here but still with a Mg, Fe-silicate, at least minimum pressures can be derived with the use of the data presented.
The Cadomian-Hercynian basement of the Massif Central, France, contains both pre-metamorphic orthogneisses (600-410 Ma) and post-metamorphic undeformed granitoids (360-275 Ma). Major-, trace-element and Sr, Nd isotopic analyses of these intrusions indicate slight differences in source material for orthogneisses and two types of underformed granitoid: (a) granodiorite/monzogranites and leucogranites. Orthogneisses have moderately enriched REE patterns (), variable HREE contents (YbN=4.2−25) and strong negative Eu anomalies (). Their ∈tNd-values are low (−5.8 to −1.7), indicating a strong crustal component in the source but ∈tSr-values are also generally low (−25 to +100) and overlap the field of age-corrected isotopic values for lower-crustal granulite xenoliths from the Massif Central. The source of the orthogneisses is therefore considered to be dominated by a lower-crustal meta-igneous component with additional input from a lower-crustal metasedimentary source.Granodiorites/monzogranites tend to show strong LREE enrichment () with small Eu anomalies () and YbN values between 6 and 8.5. These granitoids form the least evolved end-members of a series trending to highly evolved leucogranites with similar La/Yb ratios (11–36) but with generally larger Eu anomalies () and lower YbN values (3–8). Initial isotopic ratios of the leucogranites (∈tNd = −7.9 to −4.5 and ∈tSr usually > +100) mostly fall within the field of age-corrected lower-crustal metasedimentary granulite xenoliths from the region and it is proposed that these leucogranites were formed by partial melting of lower-crustal metasediments. Granodiorites and monzogranites have ∈tNd between −6.5 and −3.4 and ∈tSr-values < + 100. These values are similar to those of meta-igneous and some metasedimentary lower-crustal xenoliths and thus the granodiorites/monzogranites may have been derived from a mixture of these two components with the meta-igneous component dominating.
A plate tectonics model is presented to explain the tectonometamorphic characteristics of the European Variscides. After the closing of two oceanic domains by two-sided subduction (500-420 Ma) and obduction (420-380 Ma), collision of the European and African continental plates occurred. We propose that the subsequent complex intracontinental deformation (380-290 Ma) is the result of a double subduction of the continental lithosphere accompanied by crust-mantle décollement. This mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the Variscan Belt. Accounting for this model and for the relationships between the European Variscides and the other Paleozoic peri-Atlantic belts (Caledonides, Appalachian, Mauritanides and Morocco), we infer the relative positions of Africa, America and Europe between the Silurian and the Permian.
The polymetamorphic Moldanubian (MO) of the northeasthern margin of the Bohemian Massif has been thrust to the north onto the mainly Paleozoic sedimentary Saxothuringian of the Fichtelgebirge (FG). These two units have undergone polyphase deformation and the last regional event to affect both units was a low-pressure metamorphism in which temperatures decreased towards the north.In contrast, the nappe units of the Erbendorf-Vohenstrauss Zone (ZEV) and the Erbendorf Greenschist Zone (EGZ), which partly cover the border of the Moldanubian and the Saxothuringian, and the Münchberg nappe pile (MM), which lies on the Saxothuringian, were in parts subjected to a late medium-pressure metamorphic event.The ZEV, the EGZ, the MO and the FG are intruded by Late Carboniferous granites.Conventional K-Ar analyses, mainly of hornblendes and muscovites from the autochthonous FG and MO, the units beneath the nappes, have yielded exclusively Carboniferous dates. The oldest dates point to a regional cooling of the rocks which outcrop at the present-day surface at about 330-320 Ma, i.e., at the Early-Late Carboniferous boundary. The Late Carboniferous cooling history was largely governed by the thermal influence of the post-kinematic granites (320-295 Ma), especially in the FG and the northern MO.The high-grade metamorphic rocks in the western part of the ZEV and in the upper three nappes of the MM mostly yield dates around 380 Ma i.e., Early Devonian. The results show a relatively wide scatter. Moreover, biotites frequently appear to be older than the coexisting muscovites. Both observations indicate that the rocks underwent a later thermal influence. Whether some groups of older dates (e.g., 400 Ma) are due to excess argon or to inherited argon is still open to discussion.Slightly scattered muscovite dates around 366 Ma were obtained for the prasinite-phyllite series, one of the lower nappes of the MM. A single hornblende from the EGZ gave the same age. These two nappes have, therefore, probably been affected by a Late Devonian thermal and/or tectonic event.The muscovite dates obtained from the Paleozoic Bavarian lithofacies, the lowermost nappe of the MM∗, and the hornblende dates from the eastern part of the ZEV are indistinguishable from those of the autochthonous units FG and MO.
Fifteen chondrites, including eight carbonaceous chondrites, were analyzed for rare earth element abundances by isotope dilution. Examination of REE for a large number of individual chondrites shows that only a small proportion of the analyses have flat unfractionated REE patterns within experimental error. While some of the remaining analyses are consistent with magmatic fractionation, many patterns, in particular those with positive Ce anomalies, can not be explained by known magmatic processes. Elemental abundance anomalies are found in all major chondrite classes. The persistence of anomalies in chondritic materials relatively removed from direct condensational processes implies that anomalous components are resistant to equilibrium or were introduced at a late stage of chondrite formation. Large-scale segregation of gas and condensate is implied, and bulk variations in REE abundances between planetary bodies is possible.
Die Bedeutung von K/Ar-Datierungen an detritischen Muskoviten für die Rekonstruktion tektonomorpher Einheiten im orogenen Liefergebiet — ein Beitrag zur Frage der varistischen Krustenentwicklung in der Böhmischen Masse
- Welzel
Gliederung der redwitzitischen Gesteine Bayerns nach Stoff-und Gefiigemerkmalen. Teil I: Die Typlokalit~t von Marktredwitz in Oberfranken Geochronological studies of the Bo-hemian Massif, Czechoslovakia, and their significance in the evolution of Central Europe
- Troll
Troll G (1968) Gliederung der redwitzitischen Gesteine Bayerns nach Stoff-und Gefiigemerkmalen. Teil I: Die Typlokalit~t von Marktredwitz in Oberfranken. Bayerische Akad Wiss Abh 133:1-86 Van Breemen O, Aftalion M, Bowes DR, Dudek A, Misaf Z, Po-vondra P, Vrfina S (1982) Geochronological studies of the Bo-hemian Massif, Czechoslovakia, and their significance in the evolution of Central Europe. Trans R Soc Edinburgh: Earth Sci 73 : 89-108
Origin of Late Variscan granitoids from NE Bavaria, Germany, exemplified by Nd iso-tope systematics Subcomission on geochronology: con-vention on the use of decay constants in geo-and cosmo-chronology
- W Siebel
- A H0hndorf
- Wendt
Siebel W, H0hndorf A, Wendt I (1995) Origin of Late Variscan granitoids from NE Bavaria, Germany, exemplified by Nd iso-tope systematics. Chem Geol, in press Steiger RH, Jfiger E (1977) Subcomission on geochronology: con-vention on the use of decay constants in geo-and cosmo-chronology. Earth Planet Sci Lett 36:359-362
Two contrasting granite types Geochemistry of biotites from granitic rocks, Northern Portugal
- Chappell Bw White
Chappell BW, White AJR (1974) Two contrasting granite types. Pacific Geol 8:173-174 De Albuquerque CAR (1973) Geochemistry of biotites from granitic rocks, Northern Portugal. Geochim Cosmochim Acta 37:1779-1802
The chemical composition of biotite as an indica-tor of magmatic fractionation and metasomatism in Sn-spe-cialised granites of the Fichtelgebirge (NW Bohemian Massif, Germany) Metallo-geny of Collisional Orogens A muscovite-biotite geothermometer
- Hecht
- Leuchtenberger Zirkondatierungen
- Granit
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Hecht L (1994) The chemical composition of biotite as an indica-tor of magmatic fractionation and metasomatism in Sn-spe-cialised granites of the Fichtelgebirge (NW Bohemian Massif, Germany). In: Seltmann R, K~impf H, M611er P (eds) Metallo-geny of Collisional Orogens. Czech geol Surv Prague, pp 295-300 H/51zl S, K6hler H (1994) Zirkondatierungen am Leuchtenberger Granit/NE Bayern. KTB-Rep 94-2:B35 Hoisch TD (1989) A muscovite-biotite geothermometer. Am Mi-neral 74: 565-572
The chemical composition of biotite as an indicator of magmatic fractionation and metasomatism in Sn-specialised granites of the Fichtelgebirge Metallogeny of Collisional Orogens
- L Hecht
Wie alt ist der Weinsberger Granit? U/Pb versus Rb/Sr Geochronologie
- F Finger
- Von Quadt
Zirkondatierungen am Leuchtenberger Granit
- S Hölzl
- H Köhler
Geochemical structures in a multiple intrusion granite massif
- J Madel
Age of Karkonosze Mts. granite dated by Rb/Sr and its initial 87Sr/86Sr value (in Polish with English abstract)
- C Pin
- Mp Mierzejewski
- Jl Duthou
Der Leuchtenberger Granit und seine assoziierten magmatischen Gesteine: Zeitliche und stoffliche Entwicklungsprozesse im Verlauf der Entstehung des Nordoberpfalz-Plutons
- W Siebel
Evidence for Late Variscan emplacement of the ZEV
- E Stein
- U Kleemann