The diversity and geodynamic significance of Late Cambrian (ca. 500 Ma) felsic anorogenic magmatism in the northern part of the Bohemian Massif: A review based on Sm-Nd isotope and geochemical data
Published:January 01, 2007
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Christian Pin, R. Kryza, T. Oberc-Dziedzic, S. Mazur, K. Turniak, Jarmila Waldhausrová, 2007. "The diversity and geodynamic significance of Late Cambrian (ca. 500 Ma) felsic anorogenic magmatism in the northern part of the Bohemian Massif: A review based on Sm-Nd isotope and geochemical data", The Evolution of the Rheic Ocean: From Avalonian-Cadomian Active Margin to Alleghenian-Variscan Collision, Ulf Linnemann, R. Damian Nance, Petr Kraft, Gernold Zulauf
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Ca. 500 Ma orthogneisses and bimodal suites are widespread along the northern part of the Bohemian Massif (central European Variscides) and are interpreted to document intense magmatism during a continental break-up episode along the northern periphery of Gondwana. Based on geological setting, and geochemical and isotopic evidence, these felsic igneous rocks record the generation of: (1) magmas of pure or predominantly crustal derivation, represented by minor extrusives and much more voluminous orthogneisses similar to S-type granitoids; (2) subordinate magmas of exclusively mantle origin (ranging from within-plate alkali trachytes to oceanic plagiogranites) corresponding to felsic derivatives of associated basalts; and (3) magmas of hybrid origin, produced either as a result of large degrees of contamination of mantle-derived magmas ascending through the crust, or alternatively, generated by partial melting of mixed sources, such as interlayered sediments and mafic rocks or graywackes containing a juvenile component. The high-temperature dehydration melting process responsible for the generation of the most abundant rock-types necessitated the advection of mantle heat, in a context of continental lithosphere extension, as documented by broadly coeval basaltic magmatism at the scale of the igneous province. The large volumes of felsic magmas generated during the 500-Ma anorogenic event are interpreted to result from the combination of a hot extensional tectonic regime with the widespread availability in the lower crust of fertile lithologies, such as metagraywackes. This in turn reflects the largely undifferentiated nature of the crustal segment accreted some 50–100 m.y. earlier during the Cadomian orogeny.
Geochronological data, especially precise U-Pb zircon ages, gathered throughout the Bohemian Massif during the past 15 years have documented the existence of a widespread igneous episode of Late Cambrian to Early Ordovician age (ca. 510–490 Ma). Besides mafic and bimodal associations, copious volumes of orthogneisses document the production of large amounts of felsic magmas, the origin of which has proved to be controversial. Based on alleged broadly “calc-alkaline” geochemical features and the use of some trace element discrimination diagrams, some authors (e.g., Oliver et al., 1993; Kröner and Hegner, 1998, Kröner et al., 2001) interpreted these meta-granitoids as remnants of a batholith emplaced above a subduction zone in a continental active margin setting, related to the closure of the northern (in present-day coordinates) Tornquist Ocean separating Baltica from a supposed eastern prolongation of Avalonia. In contrast, other authors (e.g., Kryza and Pin, 1997; Crowley et al., 2000, 2002; Floyd et al., 2000; Dostal et al., 2001) emphasized the absence of intermediate (andesitic) rock-types typical of orogenic magmatism. These authors used the broad spatial and temporal association of within-plate mafic rocks to suggest that granite magmatism occurred in rift-related geodynamic environments, in a large-scale context of continental break-up leading to the opening of an ocean basin (Rheic) within the so-called “Armorica realm,” at the northern edge of Gondwana.
In this article, we present new Sm-Nd isotope results obtained from ca. 500-Ma metagranitoids from the Polish Sudetes and from coeval volcanics of the Barrandian area, and review geochemical and isotope data available on broadly contemporaneous igneous rocks from the northern part of the Bohemian Massif. These data highlight the diversity of felsic magmas produced during the ca. 500-Ma event, and can be used to put constraints on possible source materials and draw inferences on the petrogenetic processes responsible for magma generation and their tectonic setting.
GEOLOGICAL CONTEXT AND REVIEW OF GEOCHEMICAL AND Sm-Nd DATA
The location of the ca. 500-Ma igneous suites referred to in this study is shown in the geological framework of the northern part of the Bohemian Massif (Fig. 1). The geological context and major feature of the various occurrences of metagranitoids and metavolcanics are briefly described from the east (Sudetes) to the west (Saxo-Thuringian zone), based on published studies and some new Sm-Nd data (Orlica-Śnieżnik Massif gneisses; Leszczyniec metavolcanics). A short account is then given of the geological setting of the nonmetamorphic volcanics of the Křivoklát-Rokycany Complex (Barrandian), together with an assessment of published and new geochemical and isotope data.
The Orlica-Śnieżnik Massif comprises predominantly amphibolite-grade orthogneisses and staurolite-grade mica schists, and contains inclusions of high-pressure and ultra-high-pressure rocks. Supracrustal series of presumably Neoproterozoic to Cambrian age (e.g., Don et al., 1990) were intruded by large granitic plutons at ca. 500 Ma (Oliver et al., 1993; Turniak et al., 2000; Kröner et al., 2001). The subsequent Variscan tectonothermal processes were complex and included medium- to high-grade metamorphism and intense synmetamorphic deformation, accompanied by exhumation of high-grade rocks (e.g., Bröcker and Klemd 1996; Lange et al., 2002, 2005; Štipská et al., 2004, and references therein). The Orlica-Śnieżnik Massif has usually been considered to represent a gneissic dome, in which the gneisses crop out in antiforms, whereas “mantling” schists are preserved in synforms. There is some evidence, however, that the Orlica-Śnieżnik Massif is composed of a number of folded thrust sheets, as are the East Sudetic units forming a nappe pile adjacent to the southeast.
The ca. 500-Ma granitoids were transformed into ortho-gneisses known as two main varieties: the Śnieżnik and Gierałtów gneisses (e.g., Borkowska et al., 1990; Don et al., 1990, and references therein). Typically, the Śnieżnik gneisses are coarse- to medium-grained rocks with K-feldspar megacrysts, composed of quartz, microcline, oligoclase, muscovite, biotite and accessory apatite, zircon, rutile, titanite (late-stage), garnet, and opaque minerals. Local transitions to well-layered textural varieties are interpreted to reflect higher strain intensity (Turniak et al., 2000; Lange et al., 2002). In contrast, the Gierałtów gneisses are fine-grained, approximately equigranular, often thinly laminated two-mica rocks of granite composition similar to that of the Śnieżnik gneisses. Locally, they display a migmatitic texture, considerably different from the augen texture of the Śnieżnik gneisses. The Gierałtów gneisses are accompanied by lensoid bodies of amphibolites, granulites, and eclogites. Transitional textural varieties occur between the Śnieżnik and Gierałtów gneisses. Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons from both kinds of gneisses yielded ages of ca. 500 Ma, interpreted to reflect igneous crystallization from similar magmas, as also suggested by common mineralogical and geochemical characteristics (Turniak et al., 2000). Specifically, both gneiss varieties are peraluminous in character, as indicated by their mineralogy (two micas, minor garnet, and rare cordierite), the Al2O3/(CaO + Na2O + K2O) molecular ratios (A/CNK) ranging from 1.01 to 1.22, and high normative corundum, between 1.15 and 2.73. Their trace element patterns are broadly similar, with strong light rare earth element (LREE) fractionation and flat heavy rare earth element (HREE) distribution (Fig. 2); all samples display distinct negative anomalies for Nb, Sr, and Ti (Fig. 2). Both the orthogneisses were interpreted to have derived from geochemically similar protoliths (Lange et al., 2002, 2005) from mature crust, but are difficult to constrain in terms of geodynamic environments (Turniak et al., 2000).
New Sm-Nd data obtained on a subset of the samples studied by Turniak et al. (2000) are listed in Table 1. All the samples yield clearly negative initial ϵNd values (from −3.5 to −6.4), demonstrating that their source material was depleted in Sm relative to Nd on a time-integrated basis. This depletion suggests that ancient crustal materials enriched in LREE played a major role in the genesis of these rocks, as suggested by crustal residence model ages. These scatter from 1.4 to 2.1 Ga, far in excess of the emplacement age (samples MS14 and MS17, whose very high 147Sm/144Nd ratios probably reflect late-stage fractionation of a LREE-rich phase, such as monazite, were excluded from TDM calculations). Although a broad homogeneity is indicated, it can be noticed that the three Śnieżnik samples tend to have slightly higher ϵNd values than those for the Gierałtów gneisses, suggesting that the augen gneisses might have been extracted from slightly less evolved crustal sources than their equigranular counterparts. Our Nd isotope data are in general agreement with the results published by Kröner et al. (2001) for six gneiss samples from the Orlica-Śnieżnik Massif and confirm the clear crustal derivation of the ca. 500-Ma granitoids. We do not concur, however, with their interpretation in terms of “extensive melting of Precambrian basement,” nor with their suppositions on the subduction-related nature of these granitoids (see Discussion section).
Góry Sowie Massif
The Góry Sowie Massif in the central Sudetes is mostly composed of gneisses and migmatites with minor intercalations of amphibolites, as well as small bodies of high-temperature–high-pressure (HT-HP) felsic granulites and associated meta-ultra-mafic rocks (for an overview, see Kryza, 1981; Żelaźniewicz, 1990). The gneisses and migmatites comprise a wide range of textural varieties and typically contain an assemblage of quartz + plagioclase + biotite ± muscovite ± garnet ± sillimanite. Most of the gneisses and migmatites were usually interpreted as derived from sedimentary protoliths, based on field and petrological constraints (e.g., lithological variation, widespread and often abundant sillimanite). However, relatively limited rock varieties (Fig. 1), two-feldspar gneisses, are petrographically rather homogeneous and often display augen texture, and they may have derived from granites.
The geochronology of the Góry Sowie Massif rocks caused much controversy. However, common varieties of the gneisses and migmatites (layered and diatexitic types) were described as containing variable zircon populations, but with one dominant group of magmatic-type zircons (prismatic and oscillatory zoned) dated at ca. 500 Ma (Kryza and Fanning, 2007). These magmatic-type zircons were controversially interpreted either as evidence for igneous protoliths of the gneisses (Kröner and Hegner, 1998) or as representing both the igneous protoliths and sedimentary materials from reworked igneous sources (Kryza and Fanning, 2007). Representative samples of the Góry Sowie Massif gneisses and migmatites were studied for major and trace elements and Sm-Nd isotopes by Kröner and Hegner (1998), who interpreted the vast majority of the rocks as orthogneisses derived from magmatic arc–type granitoids. However, detailed geochemical and isotopic studies on the associated mafic rocks (amphibolites, coronitic metagabbros) revealed a predominance of enriched mantle-derived magmas emplaced in an extensional (probably immature rift) setting (Winchester et al., 1998; Kryza and Pin, 2002), also suggesting an extensional emplacement setting for the enclosing gneisses. The amphibolite-facies metamorphism and migmatization of the Góry Sowie Massif gneisses took place at ca. 385–370 Ma, as evidenced by isotopic ages obtained by various methods (see Kryza et al., 2004, and references therein).
In the set of samples studied by Kröner and Hegner (1998), two specimens, PL16 and PL17, represent augen gneisses of the most likely magmatic derivation. Their modal composition (quartz, K-feldspar, plagioclase, muscovite, and biotite) and major element characteristics are typical of granites, whereas the trace element distributions display strong LREE and weak HREE fractionations, with distinct negative Nb, Sr, Ti, and Eu anomalies (Fig. 2). Two other samples of layered migmatites (PL4 and PL6) and one representing diatexite (PL11) differ from the augen gneisses in having no K-feldspar in their mode but containing accessory garnet and sillimanite; their trace element patterns are roughly similar to those of the augen gneisses. In all the samples, magmatic-type zircon populations (euhedral prismatic habit and strong oscillatory zonation) of ca. 500-Ma age are common (Kröner and Hegner, 1998; Kryza and Fanning, 2007).
The Góry Sowie augen gneisses (PL16 and PL17) show ϵNd values of +0.2 and TDM of 1.13, whereas the migmatites display negative ϵ Nd values between −3.0 and −6.2, and TDM up to 1.56 (Kröner and Hegner, 1998). Three other samples from the Góry Sowie rocks were also studied for Sm-Nd isotopes by Crowley et al. (2002), giving broadly similar characteristics (unfortunately petrographic features and locations of the samples were not given).
The Izera-Karkonosze Massif in the west Sudetes is composed of the Karkonosze Granite pluton (Fig. 1), which was emplaced 328 ± 12 Ma (Rb-Sr whole-rock isochron; Pin et al., 1987), and its metamorphic envelope, which is divided by the granite into two contrasting parts: the northern part, represented by the Izera Block, and the southeastern part, which comprises metamorphic complexes of the east and south Karkonosze.
The Izera Block is composed mainly of porphyritic coarse-grained Izera granites and augen gneisses. Fine-grained granites and gneisses are minor throughout the area, whereas grano dioritic gneisses are typical of the western part of the Izera Block. All textural types of the gneisses enclose lenses of coarse-grained porphyritic granites, mostly considered as remnants of undeformed granitic protoliths. The gneisses and granites locally contain small (up to several meters thick) younger but variously deformed basic dikes.
The granitoid protoliths of the Izera gneisses gave igneous emplacement ages in the range of 515–480 Ma (Borkowska et al., 1980; Korytowski et al., 1993; Oliver et al., 1993; Żelaźniewicz, 1994; Kröner et al., 2001). The undeformed relics of these rocks correspond to the so-called “Rumburk granites” that intruded the 590–540-Ma (Żelaźniewicz et al., 2003) granodiorites of the Lusatian Massif on the west. The ca. 500-Ma granitoids of the Izera Block intruded metasedimentary and metavolcanic rocks (Berg, 1923; Achramowicz and Żelaźniewicz, 1998) of the Neoproterozoic age (640–550 Ma; Żelaźniewicz et al., 2003), which are preserved as three belts of mica schists, up to several hundred meters wide. The Izera Block suffered three deformation episodes (Mazur and Kryza, 1996; Żelaźniewicz et al., 2003), under mid- to low-grade metamorphic conditions in Late Devonian and Early Carboniferous times. In the eastern and southern Izera-Karkonosze Massif, the Kowary gneiss, dated at ca. 492–481 Ma (U-Pb ages; Oliver et al., 1993) and the Karkonosze gneiss (ca. 503 Ma; Kröner et al., 2001), both correspond to the Izera gneiss, whereas predominantly metasedimentary sequences (the so-called “Czarnów Formation” and the Velká Úpa Group) are equivalents of the mica schists of the Izera Block (Mazur and Aleksandrowski, 2001).
The Izera granitoids are typical S-type granites, as documented by petrographic features (magmatic cordierite, relict garnet, sillimanite, the lack of mafic enclaves) and geochemistry: peraluminous character (A/CNK, 1.0–1.63), high normative corundum (up to 3.5%) and relatively high 87Sr/86Sr. Although they show some geochemical similarities to syn- and postcollisional granitoids, they were considered more likely to belong, based on a range of petrographic evidence, to the anorogenic class (Oberc-Dziedzic et al., 2005a).
The ϵNd values for the Izera orthogneisses range from −5.2 to −6.9, implying that the magmas were extracted from a source with a secular enrichment in LREE. Their TDM ages, ranging between 1730 and 2175 Ma, point to an old crustal residence age of the inferred metasedimentary protoliths (Oberc-Dziedzic et al., 2005a).
In the fore-Sudetic block at the northeastern edge of the Bohemian Massif (FSB in Fig. 1), orthogneisses are known from vicinities of Strzelin and Wądroże Wielkie, in the eastern and central parts of the block, respectively. In the former area, ca. 500-Ma orthogneisses (Oliver et al., 1993; Kröner and Mazur, 2003; Oberc-Dziedzic et al., 2003b) are tectonically juxtaposed with older gneisses of ca. 600–568 Ma (Oberc-Dziedzic et al., 2003a). The younger gneisses were proved as derived from peraluminous S-type granitoids, and their trace element patterns are very similar to most of the orthogneisses described from the other areas (Oberc-Dziedzic et al., 2005b, and references therein; Fig. 2). Unfortunately, no Sm-Nd isotope data are yet available from these rocks. The orthogneisses from the other area of Wądroże Wielkie, which are petrographically similar to the Izera gneisses, have recently been dated at ca. 540 Ma (Żelaźniewicz et al., 2004), and thus, as considerably older, are not considered here in detail.
Erzgebirge (Eastern Saxo-Thuringian Zone)
The Erzgebirge, in the eastern part of the Saxo-Thuringian zone (Fig. 1), exposes various types of gneisses, traditionally grouped into red and gray varieties and interpreted as para- and orthogneisses, respectively. However, this subdivision and interpretation have appeared to be too simplified (Mingram et al., 2004, and references therein).
Referring to recent structural and petrological studies (Kröner et al., 1995; Schmädicke and Evans, 1997; Willner et al., 1997; Rötzler et al., 1998; Tichomirowa et al., 2001; Mingram et al., 2004), the orthogneisses are found within the following three tectonometamorphic units (from bottom to top): (1) medium-pressure–medium-temperature (MP-MT) gneiss unit (granite gneisses), (2) HP-HT gneiss/eclogite unit (granulite facies orthogneisses), and (3) high-pressure–low-temperature (HP-LT) mica schist/eclogite unit (granite orthogneisses). Metagranitoid bodies occur also within the so-called “transition zones” between the units mentioned above and are interpreted as large-scale shear zones (Mingram et al., 2004, and references therein).
Geochronological studies, combined with geochemical and Sm-Nd isotopic characteristics (Kröner et al., 1995; Tichomirowa et al., 2001; Mingram et al., 2004), indicate that at least two discrete magmatic events are contained in the red gneisses: (1) at ca. 550 Ma in MP-MT gneiss unit, and (2) at ca. 500–480 Ma in the high-pressure units.
Based on mineralogy and textures, the Red Gneiss Group has been subdivided into three main rock types, namely, granite gneiss, augen gneiss, and muscovite gneiss (Mingram et al., 2004). The granite gneisses vary from coarse-grained porphyritic rocks to sheared mylonitic metagranitoids. They consist of K-feldspar, quartz, plagioclase, biotite, muscovite, and small amounts of garnet. Single zircons yielded Pb-Pb protolith ages between 550 and 560 Ma (Kröner et al., 1995). The augen gneisses contain large K-feldspar porphyrocrysts within a strained matrix of plagioclase, biotite, and muscovite. Samples of augen gneisses at the rim of Reitzenhain granite yielded mean Pb-Pb ages of 551 ± 9 (Kröner et al., 1995) and 492 ± 14 Ma (Tichomirowa, 2003, in Mingram et al., 2004). The muscovite gneisses occur in the HP units and the transition zones. They are mostly fine-grained rocks composed of quartz, K-feldspar, plagioclase, white mica, garnets, and biotite (±?kyanite). Single zircons yielded a mean 207Pb/206Pb age of 479 ± 1 Ma (Kröner and Willner, 1998). Observed transitions between muscovite gneisses and metasedimentary rocks suggest that some of them could have derived from volcaniclastic deposits (Mingram et al., 2004).
Geochemically, all granite gneisses and augen gneisses are broadly similar, corresponding to peraluminous granitoids, with moderate LREE fractionation (100–200 × chondrites) relative to HREE. The muscovite gneisses are significantly different, with higher K2O/Na2O of ∼1.5, and lower Al2O3 contents (<14 wt%); their normalized REE diagrams display a spectrum of subparallel patterns with slight LREE enrichment (20–60 × chondrites) and flat HREE distribution (Mingram et al., 2004). Initial ϵNd values in all orthogneiss samples from the Erzgebirge range between −4.1 and −9.2, corresponding to crustal source with average residence times of 1.9–1.5 Ga (Mingram et al., 2004). These results lend support to the petrogenetic scenario proposed by Tichomirowa et al. (2001), in which the ca. 500-Ma granitoids might have been generated through partial melting in the deep crust of metagray-wackes and/or of ca. 540-Ma S-type granitoids.
Fichtelgebirge (Western Saxo-Thuringian Zone)
The Fichtelgebirge (northeast Bavaria) corresponds to a Late Variscan anticlinal zone in the northwestern part of the Bohemian Massif. It is mostly composed of low- to medium-grade detrital metasediments of latest Precambrian and Cambro-Ordovician age, the latter belonging to the so-called “Thuringian facies.” This facies is characterized by a para-autochtonous sequence of neritic and then hemipelagic rocks, documenting a marked tendency toward deeper water and interpreted as representing the passive margin of a continental unit to the northwest, which formed during an important Early Paleozoic phase of extension (e.g., Falk et al., 1995). Felsic meta-igneous rocks also occur and can be subdivided into plutonic and volcanic types, based on field and textural evidence (Siebel et al., 1997, and references therein). Three main plutonic bodies (Wunsiedel, Selb, and Waldershof) were studied for elemental and Sr-Nd isotope geochemistry by Siebel et al. (1997). These orthogneisses contain relictual granitic textures, with large, up to 10-cm-long K-feldspar megacrysts in the Wunsiedel gneiss. They were emplaced as discordant intrusions (e.g., Waldershof) in the upper crust, as shown by low-grade country rocks. This shallow emplacement level is also supported by the regional association with broadly similar, concordant formations interpreted as deriving from metavolcanic rocks on the basis of gradual contacts with surrounding sediments, and by the presence of bipyramidal or corroded quartz crystals (Siebel et al., 1997, and references therein). Their mineralogical composition is rather monotonous with quartz, K-feldspar, oligoclase, muscovite, biotite, and accessories (tourmaline, apatite, sphene, garnet, pyrite, zircon). A whole-rock Rb-Sr isochron age of 480 ± 8 Ma was measured for the Wunsiedel orthogneiss (Siebel et al., 1997). Metabasic rocks of similar age are rare in the Fichtelgebirge and correspond to alkaline basalts (Okrusch et al., 1989). Based on the peraluminous character (A/CNK > 1.08), high initial 87Sr/86Sr (>0.709), and negative ϵ Nd500 values (−3.5 to −6.5), Siebel et al. (1997) interpreted these felsic igneous rocks as dominantly crustal-derived melts and inferred an overall extensional setting and mantle heat input for melt generation.
Kaczawa Complex (West Sudetes)
The low-grade metamorphic Kaczawa complex in the west Sudetes crops out in several fault-bounded units, each composed of various parts of a Cambrian-Ordovician succession and sedimentary-tectonic mélange bodies. The lower part of the succession, exposed predominantly in the south, comprises composite metavolcanic suites and associated metasedimentary rocks (Baranowski et al., 1990). The metavolcanic rocks can be classified into three geochemical groups (Furnes et al., 1994): (1) transitional tholeiitic-alkaline metabasalts (pillow lavas) similar to continental rift–related magmas; (2) interlayered rhyodacitic lavas and metavolcaniclastic rocks of crustal derivation; and (3) alkaline bimodal suite of lavas and volcaniclastic rocks and alkaline metabasalts of shallow-intrusive character, also resembling initial-rift types of lavas. This part of the Kaczawa succession was interpreted to have been emplaced in a continental-rift setting during Cambrian-Ordovician times (Furnes et al., 1994; Seston et al., 2000). Preliminary U-Pb dating of zircon from a metatrachyte from that volcanic suite yielded an imprecise age of 511 ± 39 Ma (C. Pin, unpublished data). New SHRIMP zircon data (R. Kryza et al., 2005, personal commun.) confirm the ca. 500–485-Ma age of rhyodacites (associated in the field with within-plate tholeiites) and bimodal volcanics (alkali-basalts, trachytes).
The upper part of the Kaczawa succession, exposed mainly in the northern and eastern parts of the area, is composed of thick, often pillowed mid-oceanic ridge–type metabasalts and associated minor Silurian graptolite black slates and cherts and, locally, conodont-bearing Devonian slates. This part is considered to represent a more evolved rift setting and deep-basin environment, possibly developed on an oceanic-type crust during Silurian-Devonian times. The polygenetic Kaczawa mélanges, assigned mostly to the Upper Devonian and Lower Carboniferous, are interpreted as products of overlapping sedimentary and tectonic processes during the Variscan orogeny (Kryza et al., 2004, and references therein).
The rhyodacitic metavolcanics are fine-grained and often strongly cleaved rocks composed of quartz, K-feldspar, and phengitic white mica, with minor albite, chlorite, carbonate, and scarce opaque minerals. These rocks (samples N20A, OK9.8) display rather flat trace element and REE patterns, at ∼10–30 times chondrite values, with strong negative Ti and Eu anomalies (Fig. 3). The values of ϵ Nd500 for these rocks are around −3.6 (Furnes et al., 1994).
The bimodal volcanic suite of the Kaczawa complex comprises alkaline lavas ranging in composition from alkali basalts to pantellerites, with trachytes predominating. They form small domes and shallow intrusions. The felsic rocks are massive, aphanitic, and mostly aphyric, with only scarce feldspar phenocrysts and matrix composed of K-feldspar, albite, quartz, phengite, titanite, and opaques. Na-amphiboles and relict jadeite were locally ascertained (Furnes et al., 1994, and references therein). In contrast to the rhyodacites, the bimodal alkaline rocks are strongly fractionated, with high contents of most incompatible trace elements. Their REE patterns are smooth, displaying strong LREE enrichment. The ϵ Nd500 values for these bimodal volcanics are different from those for the metarhyodacites and range between +1.9 and +3.7 (Furnes et al., 1994).
Eastern and Southern Envelope of the Karkonosze Pluton
The eastern and southern parts of the Izera-Karkonosze Massif (Fig. 1) are interpreted to comprise a nappe pile formed by north-westward overthrusting of metasedimentary-volcanic Paleozoic sequences onto the Izera-Kowary orthogneisses and their envelope (Mazur, 1995; Mazur and Aleksandrowski, 2001). The structurally lower nappe, the South Karkonosze unit, is composed of volcanic and sedimentary rocks affected by ca. 360-Ma HP-LT metamorphism. The upper nappe corresponds to the epidote-amphibolite–facies meta-igneous Leszczyniec complex, formed of mafic and felsic plutonic and (sub)volcanic rocks. The northwestward nappe stacking was followed by the southeast-directed Early Carboniferous extensional collapse and ca. 330-Ma intrusion (Pin et al., 1987) of the nearly undeformed Karkonosze granite.
The volcanic suite of the South Karkonosze unit is represented by mafic and felsic lavas and volcaniclastics dated at 501 ± 8 Ma by the Rb-Sr whole-rock method (Bendl and Patočka, 1995); a broadly similar age of 485 ± 4 Ma was obtained by the U-Pb method on zircons from mafic blueschists in this area (Timmermann et al., 1999). The metavolcanics are associated with phyllites, Cambrian marbles (Hladil et al., 2003), and slates. The mafic rocks are geochemically similar to alkali basalts- to enriched tholeiites and are interpreted to have been generated in an evolved intracontinental rift setting. The felsic rocks were derived either by crustal melting or by differentiation of mantle-derived magmas with substantial incorporation of crustal melts (Patočka et al., 1997; Dostal et al., 2001).
The metaigneous Leszczyniec complex on the eastern side of the Karkonosze pluton consists of basic, intermediate, and acidic rocks, some of them dated at ca. 500 Ma by the U-Pb zircon method (Oliver et al., 1993). The mostly fine-grained mafic rocks are depleted in the most incompatible trace elements and show highly radiogenic Nd isotope signatures typical of basaltic melts extracted from strongly depleted mantle sources, such as normal-type mid-oceanic ridge basalts (N-MORBs). The felsic rocks occur as intercalations ranging from several centimeters to some hundreds of meters thick. Some of these rocks display relics of primary porphyritic textures and are interpreted to represent lavas or subvolcanic rocks. Their composition is dominated by quartz and plagioclase (<5% An), with subordinate bluish-green amphibole, epidote, chlorite, and occasional stilpnomelane (Kryza et al., 1995). Sm-Nd data obtained on representative samples of both mafic and felsic rock-types are listed in Table 2 and discussed in combination with trace element data published by Kryza et al. (1995). Two metadiorite samples (FR28 and FR29) are enriched in LREE and Th, with clear negative Nb anomalies (Fig. 3), and have ϵ Nd500 values of +1.6 and +2.8, respectively. These plutonic rocks might simply reflect crustal contamination processes or have supra-subduction zone affinities. The latter possibility seems likely for sample FR11, which has both high Th/Nb (0.18) and high ϵ Nd500 (+7.4), a feature difficult to explain in terms of crustal contamination. Bearing in mind the very complex tectonic context of this area, which might correspond to an accretionary prism, it is not clear whether these rocks belong to the same unit as the rocks dated at ca. 500 Ma by Oliver et al. (1993). The rest of the mafic samples show affinities with N-MORB, that is, depletion of Th and LREE, no significant Nb anomalies (based on X-ray fluorescence spectrometric data), and strongly radiogenic Nd isotopes (+5.6 < ϵ Nd500 < +7.9).
Vesser Area (Northwestern Saxo-Thuringian Zone)
The Vesser area (Thuringian Forest) displays the north-westernmost exposures of the Bohemian Massif. Its boundaries with basement outcrops to the south (Thuringian slate belt on the northern flank of the Schwarzburg anticline) and to the north (Ruhla Massif of the mid-German crystalline rise) are concealed by Permian deposits. The Early Paleozoic rocks consist of a ∼1200-m-thick bimodal association of volcanic, volcaniclastic, and subvolcanic rocks with minor slate intercalations, strongly overprinted by deformation and greenschist-facies metamorphism (Bankwitz et al., 1994). The igneous rocks have been studied for trace elements and Nd isotopes (Bankwitz et al., 1994). More recently, the inferred Cambrian age was confirmed by U-Pb zircon dating, ranging from 513 ± 5 Ma and 508 ± 2 Ma for dacitic pyroclastics, to 503 ± 8 and 502 ± 2 Ma for microgranitic and gabbroic, respectively, high-level intrusions (Kemnitz et al., 2002). These radiometric data reverse the succession originally proposed and document a change from early rhyolitic-dacitic volcanism to a bimodal one, with locally important mafic intrusions (Kemnitz et al., 2002). The sedimentary succession shows similarities with that of uppermost Cambrian and Tremadoc of the Saxo-Thuringian realm; it began during early, mostly rhyolitic, volcanism and evolved from shallow shelf through deep shelf to pelagic environments, reflecting the progressive flooding of an older (Cadomian) subsiding basement (Kemnitz et al., 2002). Concomitantly, igneous rocks evolved toward more mafic compositions. Metabasalts and gabbroic intrusions display variable trace element features, from early, enriched (Zr/Nb ∼8) to late-stage, depleted (Zr/Nb > 20) magmas. ϵNd values range from ∼+2 or +3 in within-plate enriched basalts to +6 in gabbros, and even +7.6 in an uncontaminated tholeiitic basalt (Bankwitz et al., 1994). This trend documents a clear temporal evolution of the mantle sources involved during the igneous event. Likewise the felsic lavas show a dramatic shift of ϵNd values from very unra-diogenic (approximately −12) in the early-stage rhyolitic volcanics, through values close to zero, to radiogenic values (+3.7) in rhyo-dacitic rocks from the bimodal association (Bankwitz et al., 1994). The combined sedimentary and igneous evidence of the Vesser zone provides a clear record of a Late Cambrian continental break-up event and the development of an intraplate rift, which probably evolved into the subsiding passive margin of a mature (Rheic?) ocean.
Křivoklát-Rokycany Volcanic Complex (Barrandian Domain)
By virtue of a very limited Variscan overprinting, the Barrandian realm in central Bohemia offers an excellent opportunity to study Late Proterozoic and Early- to Mid-Paleozoic sequences (Chlupáč et al., 1998, and references therein). In this domain, a well-defined angular unconformity separates the folded Proterozoic basement from the transgressive Cambrian or Ordovician cover. In the Přibram-Jince basin, sedimentation started in Early Cambrian times with the deposition of ∼2000 m of molasse-type conglomerates and sandstones in a continental trough. This event was followed by marine transgression in the Middle Cambrian and deposition of shales (Jince Formation), then regression and partial erosion in the Late Cambrian. Subsequent deposits of Tremadocian of the Prague basin reflect the inception of a new sedimentary cycle that lasted without major break until the Mid-Devonian (Chlupáč et al., 1998). Although volcaniclastic admixtures have been recognized in many types of Lower Cambrian deposits, neither volcanics nor volcaniclastics were recorded in the Middle Cambrian deposits (Chlupáč et al., 1998). Major magmatism occurred in the Late Cambrian, with the effusion of two large, mostly subaerial volcanic complexes, the Strašice complex in the south, mostly composed of basic and intermediate volcanics, and the Křivoklát-Rokycany complex in the north. Only the Křivoklát-Rokycany complex (Waldhausrová, 1971), covering an area of ∼180 km2, is dealt with in this work. It consists of a ∼5-km-wide northeast–southwest-trending belt, originated from fissure-type volcanoes that produced a ≤1500-m-thick sequence of flows and ash flows of mostly acid and subordinate intermediate and mafic rocks (Chlupáč et al., 1998). Based on field and petrochemical evidence, Waldhausrová (1971) distinguished four eruptive stages: the volcanism started with aphanitic dacites of maximum thickness ∼200 m (first group); large volumes of more mafic rock-types (basaltic andesites, andesites, and agglomerates) ∼500–600 m thick were then emplaced (second group); the third group is made of smaller amounts of porphyritic dacites and rhyodacites as dome-shaped bodies; these were followed by the last group, up to 800 m thick, composed only of rhyolitic lavas (mostly porphyritic, although fluidal varieties and ignimbrites also occur). A ca. 490-Ma Rb-Sr whole-rock isochron age was obtained for the Křivoklát-Rokycany complex lavas, with an initial 87Sr/86Sr of 0.7041 ± 3 (Vidal et al., 1975). This age was recently revised by zircon dating (SHRIMP) to 499 ± 4 Ma (Drost et al., 2004). These authors also provided major and trace element data documenting the affinity of rhyolites with A-type granites. On the basis of REE data, they interpreted the mafic rocks as within-plate basalts similar to enriched MORB (E-MORB). These geochemical data favor generation of the Křivoklát-Rokycany complex in an incipient rift, probably formed in a transtensional tectonic setting (Drost et al., 2004).
Additional major and trace element data, and new Sm-Nd results for seven intermediate rocks and four rhyolites are given in Tables 3 and 4. The samples of intermediate bulk composition (58–65 wt% SiO2) display homogeneous patterns on chondrite-normalized incompatible element diagrams (Fig. 3) with high abundances of Th (90–200 × chondrite) and large negative anomalies of Nb, Sr, and Ti. REE patterns (not shown) are gently sloping from La to Lu and parallel to those for more mafic samples (48–53 wt% SiO2) analyzed by Drost et al. (2004). A clear negative Eu anomaly, more pronounced in the most REE-rich samples, is observed in the intermediate samples only. The comparison of incompatible element patterns for mafic and intermediate samples (Fig. 3) highlights a close similarity (i.e., parallel patterns with deepening anomalies of Sr, Eu, and Ti with increasing silica content), suggesting a genetic relationship through fractionation of a mineral assemblage poor in Th and REE, but rich in Ti (i.e., mafic mineral[s], plagioclase, and Fe-Ti oxide). Interestingly, all the mafic samples have distinct negative Nb anomalies, at variance with the E-MORB affinity reported by Drost et al. (2004) on the basis of REE features alone. Rather, the combined REE and high-field-strength elements (HFSE) fingerprint is reminiscent of continental tholeiites (e.g., Dupuy and Dostal, 1984).
ϵNd500 values obtained for intermediate rocks are all clearly positive (Table 3), implying ultimate derivation from a mantle reservoir with time-integrated depletion of LREE. With one exception (KRB-8, with ϵNd500 = +2.6), Nd isotopes show little variation over a range of silica content (58.5–65.3 wt%), and the most radiogenic value (+5.3) is measured in the most silicic sample (KRB-4a, with 65.3 wt% SiO2). This observation is not in line with a simple assimilation–fractional crystallization (AFC) model to explain the isotopic variation within intermediate rocks, but rather suggests that heterogeneous sources and/or different evolutionary paths of magma batches were involved. The rhyolites have much lower, albeit still positive, ϵNd500 values, from +0.2 to +1.8. These values preclude derivation from ancient crustal rocks with time-integrated LREE enrichment, in keeping with the relatively low Sr initial ratio (∼0.704; Vidal et al., 1975). Our data are still too limited to draw firm inferences on the petrogenesis of these rhyolites. Samples KRB-8 (65 wt% SiO2, ϵNd500 = +2.6) and KRB-5a (74 wt% SiO2, ϵNd500 = +1.8) suggest that at least some of the rhyolites might have evolved from mantle-derived, mafic to intermediate parental magmas, through AFC in a crustal magma chamber. Alternatively, the rhyolites or their precursor might have been produced in the lower crust, through magma mixing between mantle and crustal end-members, or generated directly through partial melting of sources that were characterized by Nd isotopes intermediate between those of depleted mantle and typical continental crust. Based on the geochemical affinity of rhyolites with A-type granitoids (Drost et al., 2004), possible crustal sources might include igneous rocks of tonalitic to granodiorite composition (e.g., Creaser et al., 1991) or partially dehydroxylated biotite- and amphibole-bearing gneisses (e.g., Skjerlie and Johnston, 1992). Provided they occurred at that time in the Barrandian lower crust, Neoproterozoic graywackes (e.g., Jakeš et al., 1979) might provide suitable source materials, as suggested by preliminary Nd isotope data (ϵNd500 ≈ −1). In this scenario, under- and intraplating by basaltic magmas similar to continental tholeiites (as represented by early-stage and volumetrically subordinate mafic rocks in the Křivoklát-Rokycany complex) would have caused HT metamorphism and partial dehydroxylation of the Barrandian lower crust, followed by dehydration-melting to produce rhyolitic magmas. The AFC and partial melting models are not mutually exclusive and might have operated during the same igneous event.
Main Features and Inferred Source Materials
Characteristically, the numerous occurrences of ca. 500-Ma gneisses from the northern part of the Bohemian Massif consist of leucocratic rocks that display two main varieties: (1) coarse- to medium-grained rocks with K-feldspar megacrysts (augen gneisses; e.g., Śnieżnik gneisses, most Izera metagranitoids, some Red Gneisses from eastern Erzgebirge, Wunsiedel gneisses); and (2) finer-grained, equigranular rocks, often more intensely deformed (e.g., Gierałtów gneisses, some Izera metagranites, some Red Gneisses from Erzgebirge), but transitional textural varieties are also observed, at least in part caused by heterogeneous ductile deformation. The emplacement level of igneous precursors is often unconstrained because of the intense tectono-metamorphic overprinting, having reached eclogite- or granu-lite-facies conditions in several cases (Gierałtów, Góry Sowie, Red Gneisses). However, relatively shallow levels of intrusion can be inferred in certain places (e.g., Izera, Waldershof gneiss) where Variscan deformation and metamorphism were limited, in general, to low-grade conditions. Although variable in terms of textures (porphyritic versus equigranular) and degree of ductile deformation, all these metagranitoids are leucocratic, muscovite-bearing rocks that share an important common feature, namely, the absence of dark, microgranular enclaves. Importantly, they are not associated with intermediate (dioritic) or mafic lithologies, except rare metabasite bodies (sometimes eclogitized) interpreted to reflect boudinaged basaltic dikes emplaced after—and unrelated to—granite magmatism. The mineralogical composition of the gneisses is broadly homogeneous and monotonous with quartz, K-feldspar, relatively sodic plagioclase, muscovite, and biotite, documenting their origin as two-mica granites. In most occurrences (except the Izera gneisses, for which prismatic cordierite is widespread), highly aluminous minerals (cordierite, garnet, sillimanite) are scarce or even absent, only primary muscovite reflecting slightly peraluminous igneous protoliths.
The major element composition of these granitoids (e.g., Borkowska et al., 1990; Kröner et al., 1995; Siebel et al., 1997; Turniak et al., 2000; Mingram et al., 2004; Oberc-Dziedzic et al., 2005a,b) shows high contents of silica (68–77 wt%), alumina (12–15 wt%), and alkali elements (7–9 wt% Na2O + K2O); low concentrations of Ca, Mg, and Fe; and a variably pronounced peraluminous character (A/CNK > 1). Although some mobility of largeion lithophile elements certainly occurred during deformation and metamorphism, implying that these elements should be used with caution, K2O > Na2O in all cases, and CaO contents rarely exceed 1.5 wt%. Overall, these rocks are reminiscent of S-type granitoids (e.g., Clemens, 2003, and references therein).
Chondrite-normalized patterns for selected trace elements considered as relatively immobile (except Sr) during deformation and metamorphism (Fig. 2) display similar features in all examples studied, specifically, strong enrichment in Th (typically 100–400 × chondrite abundance) and LREE compared to middle REE and HREE. Typically, the LREE abundances decrease with increasing silica content, or with increasing Rb/Sr ratio used as a differentiation index. Concomitantly, the negative Eu anomaly deepens markedly, and the degree of fractionation of the HREE decreases, as shown by the transition from steep to almost flat chondrite-normalized HREE patterns in the more evolved samples (e.g., Fichtelgebirge orthogneisses; Fig. 2). This difference may be interpreted to reflect the fractionation, during magma differentiation, of a mineral assemblage containing plagioclase and one or several accessory phases rich in LREE and middle REE relative to HREE. In some cases (e.g., Izera, Wunsiedel), the degree of LREE enrichment decreases in the most evolved samples, giving flat to LREE-depleted patterns, probably as a result of monazite fractionation. Deep negative anomalies of Nb, Ti, Sr, and Eu occur throughout. Although crystal fractionation (e.g., plagioclase for Sr and Eu) certainly played a role, these trace element data are interpreted to reflect derivation from source materials that were themselves enriched in Th and LREE, and relatively depleted in Sr and elements of the Ti group, as typical for granitoids, quartzofeldspathic gneisses, pelitic and graywackeous mature sediments, and crustal materials in general (e.g., Taylor and McLennan, 1985). The distinct Ti-Nb negative anomalies and HREE fractionation shown by the least evolved samples further suggest that the primary magmas separated from a rutile- and garnet-bearing restitic assemblage, as expected for granulitic residues left behind after partial melting of most metasedimentary sources. It is likely, however, that the negative Nb anomalies observed in the 500-Ma granitoids were in great part inherited from their crustal source materials.
Radiogenic isotope data provide important, albeit not unequivocal, constraints on the possible source materials involved. Although late-stage disturbances of the Rb-Sr system occurred in many cases during metamorphism and deformation, whole-rock Rb-Sr isochrons broadly preserved igneous emplacement ages in some occurrences, as shown by reasonably good agreement with U-Pb zircon data. In these cases, initial 87Sr/86Sr ratios are fairly radiogenic (0.715 ± 5 for Gierałtów gneisses, Borkowska et al., 1990; 0.709 ± 1 for Izera granitoids, Borkowska et al., 1980; 0.7095 ± 7 for the Wunsiedel orthogneiss, Siebel et al., 1997) and point to source materials with relatively high time-integrated Rb/Sr ratios, such as relatively mature upper crustal sediments. This possibility is further corroborated by largely negative ϵNd values (Fig. 4) throughout (mean −5.0, standard deviation [SD] 1.0 for seventeen samples from the Orlica-Śnieżnik dome, Kröner et al., 2001, and this study; approximately −6 for two samples from Góry Sowie, Kröner and Hegner, 1998; mean −5.5, SD 0.7 for thirteen samples from Izera, Kröner et al., 2001, and Oberc-Dziedzic et al., 2005a; mean −5.1, SD 1.8 for five samples from Erzgebirge Red Gneisses, Kröner et al., 1995; mean −3.5, SD 0.3 for seven samples from Fichtelgebirge, Siebel et al., 1997). These values reflect extraction from source reservoirs enriched in LREE (i.e., with low Sm/Nd) on a time-integrated basis. Average crustal residence ages have been calculated (or recalculated, for the sake of homogeneity) relative to the island-arc type depleted mantle model of DePaolo (1981). Samples with 147Sm/144Nd > 0.15 were discarded from these calculations, to keep to a minimum the effect of late-stage increases in the Sm/Nd ratio, probably caused by monazite fractionation, which would lead to spuriously old model ages. The results range from 1.2 to 2.1 Ga, with a majority of values falling between 1.5 and 1.8 Ga. Bearing in mind the widely acknowledged S-type character of most 500-Ma granitoids (e.g., Siebel et al., 1997; Turniak et al., 2000; Tichomirowa et al., 2001; Mingram et al., 2004; Oberc-Dziedzic et al., 2005a,b), these dates should not be taken as evidence, even circumstantial, for derivation from Mid-Proterozoic crust, and more especially, as a reason to infer the existence of any ancient basement at depth. Instead of referring to specific crustal formation event(s), the TDM ages most likely reflect the average of several detrital components mixed in the granitoid sources, insofar as no significant mantle material was added at the time of magma generation. This interpretation is supported by complex age spectra of inherited zircons that occur frequently in these rocks. For example, a 2.1-Ga inherited component was documented in the Rumburk granite (Oliver et al., 1993), and zircon grains up to 2.5–2.6 Ga have been found elsewhere (e.g., Kröner et al., 1995; Turniak et al., 2000; Kryza and Fanning, 2007). These point to the presence of old, most likely multiply recycled, crustal components in the source of the ca. 500-Ma granitoids. Geologically more interesting is the occurrence, documented by SHRIMP measurements (Turniak et al., 2000), and possibly, by Pb/Pb evaporation ages (Kröner et al., 1995), of inherited igneous zircons as young as ca. 540 Ma in some cases. These zircons document the young depositional age (and/or intrusion age) of the volcaniclastic sediments (and/or S-type granitoids), which formed the probable source materials of the 500-Ma orthogneisses.
In summary, the combined radiogenic isotope data suggest that most of these granitoids correspond to partial melts of heterogeneous sedimentary sources containing both ancient (2 Ga and/or older) mature, recycled components, and recent erosion products from Late Proterozoic igneous sources. The relative contribution of these two contrasting sedimentary components to the granitoid source material is likely to vary from place to place. For example, the old recycled source (metapelites?) probably played an important role in the genesis of the strongly peraluminous Izera granitoids, with ϵNd values of approximately −5 to −7, and TDM in the range of 1.73–2.17 Ga (Oberc-Dziedzic et al., 2005a). In contrast, less mature sediments, such as graywackes containing a substantial igneous component of Late Proterozoic age in addition to old recycled epiclastic materials, were probably more important in the source of the Wunsiedel orthogneiss (ϵNd approximately −3; Siebel et al., 1997) or some of the Red Gneisses (ϵNd approximately −3 to −4; Kröner et al., 1995).
Bearing in mind the possible disturbances of Si, Na, K, and Ca during Variscan deformation and metamorphism and the likelihood of some restite entrainment (Al, Fe, Mg) in anatectic melts, the major element composition of the ca. 500-Ma orthogneisses fits rather well with the range of melts experimentally produced from most common crustal rocks in the absence of fluids. Indeed, except under special circumstances, the amount of free water available in the lower crust is expected to be too low to make H2O-saturated melting a significant process (e.g., Thompson and Connolly, 1995; Clemens and Watkins, 2001). In broad terms, detrital sedimentary rocks, such as metapelites and metapsam-mites (graywackes and quartzofeldspathic gneisses), are the most fertile protoliths for dehydration melting, because they contain appropriate relative proportions of quartz and feldspar on the one hand, providing a low-temperature melting fraction, and micas on the other hand, providing H2O for lowering the melting temperature (Thompson, 1996, and references therein). Although potentially fertile in terms of quartzofeldspathic components, common granites and granodiorites are too anhydrous (i.e., do not contain enough biotite + muscovite) to provide much granitoid magma through remelting (e.g., Patiño Douce and Johnston, 1991). Strongly peraluminous granitic melts could be produced from amphibolite protoliths (Patiño Douce and Beard, 1995), but such source rocks are considered rather unlikely in the present case, based on high potassium contents, radiogenic Sr isotopes, largely negative ϵNd values, and the widespread occurrence of inherited zircons in the studied orthogneisses, which instead favor a metasedimentary source. According to Thompson (1996), the interpretation of experimental results indicates that pelites (containing more mica than feldspar and quartz) would be more fertile than psammites (containing more feldspar and quartz than micas) at low P (e.g., 5 kilobars) and when the plagioclase is calcic (e.g., An40–An50). In contrast, psammites would be more melt productive than pelites at higher pressure and when plagioclase is more sodic. For these reasons, it is considered that the optimum proto-lith for lower crustal anatexis is close to a metagraywacke (e.g., Clemens and Vielzeuf, 1987; Patiño Douce and Johnston, 1991; Thompson, 1996), because of its large plagioclase (providing Na2O) and quartz contents, and relative scarcity of aluminosilicate. Together with geochemical and isotope data, these considerations give credence to a scenario explaining the generation, ca. 500 Ma ago, of large volumes of granitic magmas through partial melting of highly fertile metasedimentary materials, such as the Neoproterozoic graywackes, which form a significant proportion of the outcrops of the northern Bohemian Massif.
In contrast to the broad compositional and isotopic homogeneity of the orthogneisses, the coeval felsic volcanics or hypabyssal intrusions show a great range of elemental and isotopic variation. At one extreme, some rhyolites or dacites display extremely nonradiogenic Nd isotopes, which provide evidence for interpreting them as almost pure crustal melt. The typical example for that group is provided by an early-stage, rhyolitic lava from the Vesser area (sample 7904 from Bankwitz et al., 1994) with an initial ϵNd value as low as −11.7. A marked depletion of the HREE (<10× chondrite abundance) suggests that the parental magma separated from a garnet-rich residuum in the lower crust. The TDM model age calculated for this sample (1.9 Ga) suggests that its parental magma might have been generated through partial melting of ca. 2-Ga basement rocks, such as those found as small inliers in northern Armorican Massif (Icartian orthogneisses), or from sedimentary detritus eroded from a similar source, without significant addition of younger components.
More widespread are felsic lavas with less extreme, but still strongly negative, ϵNd500 values—for example, volcani clastic samples with ϵNd500 of approximately −5 in the Świerzawa unit, and with ϵNd500 of approximately −3.7 in the Bolków unit, both from the Kaczawa Mountains (Furnes et al., 1994). Felsic metavolcanics (porphyroids) from the east Karkonosze complex—with ϵNd500 values from −4.3 to −5.5 and TDM ranging from 1.5 to 1.8 Ga (Dostal et al., 2001)—also belong to this predominantly crustally derived group, although the relatively low 87Sr/86Sr initial ratios measured for these rocks (Bendl and Patočka, 1995) suggest that the Sr budget in the parental magma was not dominated by an ancient, mature component with a high time-integrated Rb/Sr ratio. The concordant gneisses from the Fichtelgebirge, interpreted as metavolcanics, can be ascribed to the same group, based on their distinctly negative ϵNd500 values (from −3.8 to −6.4, mean −4.9, SD 1.0, for seven samples), and model ages ranging from 1.5 to 1.8 Ga (Siebel et al., 1997).
At the other extreme are rhyolites and trachytes, closely associated in the field with volumetrically predominant meta-basalts of various types. In the southern Kaczawa Mountains and Vesser area, felsic lavas, such as sample 7525 from Vesser (Bankwitz et al., 1994) and samples Swie 4, P4.1, C14.15, and LA2 from Kaczawa (Furnes et al., 1994), are associated with metabasalts similar to ocean island basalts and alkali basalts. Based on elevated concentrations of LREE, Zr (600–1200 ppm), and Nb (>100 ppm), these lavas are similar to alkaline rhyolites and trachytes. They share the same Nd isotope signature (specifically, ϵNd500 of approximately +2 to +4) as the associated enriched basalts and are interpreted to represent evolved liquids left after closed-system differentiation of the concomitant within-plate basalts. The fractionation of HREE in these alkaline rhyolites mirrors that of the associated basalts. Conspicuous negative anomalies of Sr, Eu, and Ti suggest that low-pressure fractional crystallization of plagioclase + Fe-Ti oxide + mafic phase(s) was probably involved in this differentiation process, leading to a general increase of REE concentrations.
In the Leszczyniec unit of the east Karkonosze complex, felsic rocks of plagiogranitic mineralogical composition occur in close association with Th- and LREE-depleted mafic rocks strongly reminiscent of N-MORB (Kryza et al., 1995). Geochemically, the felsic rocks are characterized by low potassium contents and almost flat chondrite-normalized patterns of incompatible elements, with the exception of Sr, Eu, Nb, and Ti, which show deep anomalies. Sm-Nd analyses (Table 2) of these samples yield very radiogenic isotope signatures (ϵNd500 from +6.2 to +6.8) within the range of values measured for local mafic to intermediate volcanics (from +5.6 to +7.9). Based on these data, the Leszczyniec felsic rocks resemble oceanic plagiogranites, generated either by fractional crystallization of a basaltic parent, or by low-degree melting of a similar protolith. Deciphering the specific petrogenesis of these rocks is beyond the scope of this article, but the occurrence of a few intermediate samples (FR11 and FR31) with 55 and 64 wt% SiO2 and ϵNd500 +7.4 and +6.2, respectively, might lend some support to the fractional crystallization model.
Besides those felsic rocks which clearly prove to be either largely of crustal origin (as shown by strongly negative ϵNd) or genetically linked with the enriched, within-plate type (ϵNd approximately +3), or depleted (ϵNd > +6) basalts associated in the field, several occurrences show acidic magmas with intermediate Nd isotope signatures. This is the case of the Křivoklát-Rokycany rhyolites (Table 4), which have ϵNd values close to zero for three of four analyzed samples. As another example, two samples from the Vesser bimodal suite, containing 58 and 71 wt% silica, have ϵNd500 values of −0.5 and +0.2, respectively (Bankwitz et al., 1994). These Nd isotope features indicate that the average source of these rhyolites was neither enriched nor depleted in terms of time-integrated Sm/Nd ratio.
This almost chondritic Nd isotope signature is open to several geological interpretations. First, it might reflect closed-system differentiation from a mafic magma extracted from a source with broadly chondritic Sm-Nd characteristics, as documented for many continental flood basalt suites (irrespective of the specific explanation given to this observation). Although such basalts have not been documented in the Vesser area, this hypothesis cannot be totally dismissed, bearing in mind the poor preservation potential of these subaerial rock types. Note in this respect that two samples of ca. 490-Ma orthogneisses from the high-grade Góry Sowie analyzed by Kröner and Hegner (1998) have initial ϵNd values of +0.2, similar to some of the mafic plutonic rocks (metagabbronorites) of the same domain, which are geochemically similar to continental tholeiites (Kryza and Pin, 2002).
Second, the ϵNd values close to zero might reflect igneous mixing, in adequate proportions, of typical continental material (with negative ϵNd values) and a mafic magma extracted from a time-integrated depleted mantle source (i.e., with positive ϵNd), such as those occurring in the Vesser suite (ϵNd500 up to +7.6; Bankwitz et al., 1994) and in the Křivoklát-Rokycany complex (ϵNd500 approximately +5). This mixing might have occurred through crustal contamination during ascent through, and/or storage in, the crust via bulk assimilation or AFC processes. Such a scenario may be supported, in the Křivoklát-Rokycany complex, by the occurrence of an andesite with ϵNd500 +2.6 and a rhyolitic sample with ϵ Nd500 +1.8, because these two samples appear to bridge the gap between mafic and felsic rocks and possibly document an AFC process. Attempting to model quantitatively these inferred mixing processes is beyond the scope of this review and would remain a rather academic exercise, insofar as the potential crustal end-members are very poorly constrained in terms of Nd concentrations and isotope signatures.
Alternatively, the Sm-Nd data might be satisfied readily if a source with suitable isotope characteristics occurred at depth in the local crust and was able to partially melt during the ca. 500-Ma episode. This scenario might have involved anatexis of interlayered amphibolite and peraluminous metasediments, as experimentally studied under lower crustal conditions by Skjerlie and Patiño Douce (1995). In another interpretation, the mixed source might have been generated during the erosion-sedimentation cycle. This hypothesis cannot be a priori dismissed for the Křivoklát-Rokycany complex rhyolites, based on scarce Nd isotope data on Late Proterozoic graywackes of the Barrandian area, which point to ϵNd500 values near −1 (Pin and Waldhausrová, unpublished data). In such a scenario, the mafic end- member (basalts and andesites) of the Křivoklát-Rokycany complex might represent magmas extracted from a hydrous upper mantle domain inherited from Late Proterozoic subduction processes, whereas the rhyolites would mainly reflect crustal melting of a hybrid source (graywackes) generated, several tens of Ma earlier, by sedimentary mixing in a Late Proterozoic basin that trapped both volcanogenic detritus from juvenile sources and old recycled clastic components.
In conclusion, extrusive silicic magmas occurring either alone or, more commonly, as part of broadly bimodal mafic-felsic associations, include:
Rhyolites of pure or predominantly crustal derivation, representing, at least in part, the extrusive counterpart of the much more voluminous orthogneisses;
Rhyolites or trachytes, and even plagiogranites, of exclusively mantle origin, corresponding to felsic derivatives of abundant, associated enriched or depleted basaltic magmas; and
Rhyolites of inferred hybrid origin, generated either as a result of a high degree of crustal contamination of mantle-derived magmas ascending through the crust, or by partial melting of mixed sources (e.g., interlayered sediments and mafic rocks, or graywackes consisting of a sedimentary mixture of epiclastic components of ancient crustal origin and juvenile components fed by the erosion of mantle-derived, broadly coeval igneous rocks).
Causes of Crustal Melting
The generation of copious volumes of magma from crustal protoliths is well documented by the geochemical—and, particularly, Nd isotope—features of the ca. 500-Ma orthogneisses and some of the broadly coeval felsic metavolcanics. This crustal derivation prompts the question of the heat source for partial melting. Although the existence of certain low-temperature granitoids can be inferred (e.g., Miller et al., 2003), it is generally accepted that S-type felsic magmas erupted or emplaced at shallow crustal level were H2O undersaturated and generated in the lower crust through HT melting. Melting temperatures >800 °C are indicated by geothermometry studies (see references in Clemens, 2003), in agreement with the 850–900 °C range of dehydration melting experiments. Such elevated temperatures and the abnormally high heat flow they imply cannot be reached by crustal thickening, but require advective heat input from the mantle (e.g., Thompson, 2000). Therefore, it is concluded that mantle was the source of the excess heat responsible for widespread crustal melting during the 500-Ma event. More specifically, it is inferred that hot asthenosphere uprising during progressive stretching of the overlying lithosphere provided both an increased basal heat flow and basaltic partial melts, which underplated and intruded the continental crust, thereby causing copious partial melting of fertile lithologies. This scenario is supported by the occurrence of coeval basaltic magmatism in many of the occurrences discussed in this work and the recognition that periodic, multiple intrusions of basaltic magmas over a time span of a few hundred thousand years provides a very efficient way to promote partial melting in the lower crust (e.g., Petford and Gallagher, 2001).
Inferred Tectonic Setting
Continental lithosphere extension provides a suitable mechanism to trigger granulite facies and partial melting in the lower crust at a regional scale, particularly when asymmetric extension and crustal-scale detachments are involved (Sandiford and Powell, 1986). Indeed, two favorable conditions promoting partial melting of the lower crust are combined in extensional settings, specifically, decompression through dehydration melting reactions and increased heat supply through the crust-mantle boundary caused by lithospheric mantle thinning. The large-scale spatial and temporal association of crustal melts with mantle-derived magmas typical of rifting contexts further demonstrates that intrusion of mafic magmas occurred in many places and allowed for convective heat transfer into the surrounding crust. The combination of a “hot” tectonic setting with the presence of lithologies characterized by high melt productivity (Neoproterozoic graywackes) favored the generation of mobile magmas that emplaced as high-level granites (now orthogneisses) or erupted as lavas. A large-scale rifting context leading to continental breakup is independently indicated by the sedimentary record of the Late Cambrian–Early Ordovician wherever the Variscan tectono-metamorphic overprinting was not too strong (e.g., Falk et al., 1995; Kemnitz et al., 2002). The occurrence of such examples as the Křivoklát-Rokycany complex, with undeformed mafic rocks similar to continental tholeiites and felsic rocks showing affinities with A-type granitoids, demonstrates that the rifting event failed in some cases, merely producing a shallow marine basin. This failure allowed the ca. 500-Ma volcanics to escape tectonometamorphic overprinting during Variscan events. Besides this example of aborted break-up, the temporal evolution of igneous rocks toward N-MORB (Vesser) suggests progressive rifting, evolving toward oceanic spreading. Independently, the relics of HP metamorphism documented in several other occurrences indicate that some rifted segments were brought down to mantle depths during their subsequent evolution. This HP imprint is interpreted as a record of subduction of former passive margins, following consumption of negatively buoyant, attached oceanic lithosphere. Along with scarce ophiolitic relics (e.g., Pin, 1990), this subduction process demonstrates that the 500-Ma rifting episode reached an ocean-spreading stage.
The term anorogenic might convey the false connotation that rifting occurred in the middle of a large stable craton. Instead, the rifted domain was situated within a broad band of relatively juvenile continental lithosphere rimming the northern edge of the north African (Gondwana) cratonic domain. This lithosphere was largely newly formed before and during the Cadomian (pan-African) orogeny, probably as the result of long-lived igneous and sedimentary accretion in a Pacific-type active margin setting. We do not concur, however, with authors supporting a still-active subduction as the driving mechanism for the 500-Ma rifting and break-up event. It seems more likely that active subduction ceased significantly earlier, either before Cadomian tectonics through switching to a transform regime (cf. Nance et al., 1991), possibly following a ridge-trench encounter, or at the time of Cadomian collisional processes (as the final result of oblique convergence), some 50 Ma before the major break-up episode. Igneous rocks emplaced during the intervening time span, which might be interpreted conventionally as late Cadomian magmas, consist of ca. 540-Ma I-type granodiorite plutons of crustal derivation (as shown by Nd isotopes) in Lusatia and eastern Erzgebirge (e.g., Korytowski et al., 1993; Kröner et al., 1994; Linnemann et al., 2000; Tichomirowa et al., 2001; Dörr et al., 2002) on one hand, and ca. 510–520-Ma I-type granitoids and gabbros in the Teplá-Barrandian domain (e.g., Dörr et al., 1998, 2002) on the other. Whatever the geodynamic cause(s) for these older igneous events, the change from I-type to S-type sources for granitoid magmas during the Cambrian period might reflect sequential partial melting events of a vertically zoned late- to post-Cadomian crust, mostly composed of pre-Cadomian metaigneous materials at deeper levels, overlain by a thick sedimentary pile including both Late Proterozoic graywackes and Cambrian clastics. In a very tentative interpretation, the ca. 540-Ma magmas and coeval LP-HT metamorphism (Zulauf et al., 1999) might have been generated as the result of slab break-off (cf. Atherton and Ghani, 2002) following the Cadomian collision. Based on the occurrence of very thick deposits accumulated in shallow, rapidly subsiding depressions under continental, then marine, conditions (Chlupáč et al., 1998) and structural data pointing to dextral transtension (Zulauf et al., 1997), it is likely that a rift-related regime prevailed throughout the Cambrian system in the Barrandian domain. For this reason, and in the lack of detailed geochemical and isotope data, it is considered that the ca. 510–520-Ma plutons of the Teplá-Barrandian record an early igneous pulse within the broader context of a protracted period of oblique extension, as already proposed by Zulauf et al. (1997).
Active rifting triggered by the impingement of a mantle plume on the lithosphere is a popular model, which was suggested to explain the Cambro-Ordovician rifting in the Bohemian Massif (e.g., Floyd et al., 2000). However, it is not possible to clearly document a deep-mantle plume origin for rift-related basalts on geochemical grounds, and we are reluctant to invoke a “plume” as the best explanation for mantle melting and continental break-up. Indeed, purely plate-tectonic processes leading to so-called “passive” rifting and continental break-up are probably sufficient causes to generate significant melts from the astheno-sphere (e.g., Smith and Lewis, 1999; Anderson, 2000; Favela and Anderson, 2000). No large excess temperature (i.e., no plume) is required if relatively fertile mantle underlay the stretched, rifted lithosphere. We suggest that this was indeed the case beneath the newly accreted Cadomian crustal domain.
Beyond their geodynamic bearing on the evolution of the continental lithosphere in the Bohemian Massif, our results have broader implications on two general issues, namely, the debated status of “A-type granitoids” and the characterization of the inferred tectonic setting of granitoids through the use of geochemical discrimination diagrams.
This review demonstrates that highly contrasting silicic magmas may be generated, broadly concomitantly, in a single province of medium size (∼500 km across) under a similar extensional tectonic regime. These magmas, ranging from crustally derived, metaluminous to peraluminous granitoids, to mantle-derived, sometimes peralkaline silicic magmas, through rock-types with intermediate characteristics, can be safely considered as anorogenic granitoids, although only a minority displays “Atype” geochemical characteristics as usually defined (Collins et al., 1982; Whalen et al., 1987). It has been known for decades that subalkaline or even peraluminous silicic igneous rocks occur in certain rifting environments and form a distinct group of ano-rogenic granitoids (e.g., Hanson and Al-Shaieb, 1980; Anderson and Thomas, 1985; Finger et al., 2003), besides the typical “Atype” granitoids or “within-plate granites” (Pearce et al., 1984). This observation highlights the fact that A-type granitoids, defined by both geochemical characteristics and inferred tectonic setting, are only an end-member among a much larger class of rocks, ranging from peraluminous to peralkaline, as shown in this study. Typical A-type granitoids are commonly interpreted as partial melts of lower crustal rocks (felsic granulites) that suffered earlier melt extraction (e.g., Collins et al., 1982) or merely H2O loss during a metamorphic event (Skjerlie and Johnston, 1992). Alternatively, they might represent partial melts of crustal igneous rocks of tonalitic to granodioritic composition (Creaser et al., 1991). In other cases (mafic-felsic bimodal associations), they correspond to differentiates of basaltic magmas (e.g., Eby, 1992) or to partial melts from underplated mafic bodies (e.g., Poitrasson et al., 1995). In any case, unusually high temperatures are required to trigger partial melting of relatively refractory source rocks, implying the involvement of thermal ± mass input from underlying mantle, as commonly occurs during lithospheric extension. This involvement accounts for the systematic association of A-type magmas with rift-related settings. However, if the rifted continental crust is relatively immature and contains fertile rock-types at depth, as inferred for the northern Bohemian Massif at 500 Ma, partial melting of such lithologies is inescapable, thereby producing relatively large volumes of peraluminous to metaluminous granitoids. This first style of anorogenic granitoids would be generated at an early stage from the most fertile lithologies present in the melting domain. At a later stage, more typical A-type magmas could be produced from more refractory lower crustal lithologies, provided that sufficient input of heat allows these rocks to melt. A-type granitoids are anticipated to occur alone in cratonic segments characterized by an ancient, relatively refractory lower crust. Conversely, their association with meta- or peraluminous granites would characterize less-differentiated crustal segments that contain fertile lithologies.
As already emphasized (e.g., Creaser et al., 1991), the A-type granite terminology is questionable because of its inherent ambiguity (geochemistry versus tectonics), and classifications based on factual mineralogical-geochemical criteria, and/or on inferred sources (e.g., the I-S scheme) seem preferable. In this framework, the ca. 500-Ma felsic magmas consist mainly of S-type, with minor I-type and/or even M-type. Diverse petrogenetic processes were involved, including differentiation from mantle magmas and partial melting of various crustal source materials. This multiplicity of processes gave rise to the observed geochemical and isotopic diversity, but whatever their chemical fingerprint, all these rocks were generated in an extensional tectonic regime in the broader geodynamic context of continental break-up. For this reason, they can all be termed anorogenic granitoids, despite their great diversity of source materials and petrogenetic mechanisms.
Following a common practice in the study of ancient basaltic rocks, geochemical discrimination diagrams are often used to infer ancient tectonic environments of granitic rocks. This approach is potentially very misleading, as can be shown by the plotting of the vast majority of the ca. 500-Ma felsic rocks within the “volcanic arc granite” and “collision granite” fields, in contradiction to the typical within-plate affinity of associated basaltic rocks and geological constraints, wherever these have not been erased by Variscan overprint. This failure of chemical discrimination diagrams is interpreted to reflect the fact that granite magmas mainly mirror their (mostly crustal) sources and do not have any simple geochemical relationship with the geodynamic setting prevailing at the time of their genesis (see discussion in Oberc-Dziedzic et al., 2005a). Indeed, chemically similar granitic magmas could be produced by broadly similar degrees of partial melting of similar source materials, irrespective of the local geodynamic setting, provided that melting can occur. Rather, the “volcanic arc” signatures commonly found in the ca. 500-Ma orthogneisses are interpreted to reflect the ultimate provenance of their inferred sedimentary source, specifically, Late Proterozoic graywackes, which contained a significant contribution from subduction-related igneous sources (e.g., Jakeš et al., 1979; Drost et al., 2004). If this interpretation is accepted, the geochemical discrimination diagrams for the 500-Ma granitoids are biased by inheritance, and they do not convey any useful information on the tectonic setting at the time of magma generation. Clearly, ancient geodynamic settings should be inferred from a combination of various types of geological evidence, among which granite geochemistry should be used with extreme care.
This review highlights the diversity of rock-types and inferred source materials involved in felsic magmatism during the ca. 500-Ma event. Based on converging lines of evidence, including the geochemistry of concomitant basalts, the tectonostratigraphic context, and the igneous rock association, an extensional regime is clearly documented, as already emphasized by several previous studies. The 500-Ma igneous event is therefore interpreted to be unrelated to any active subduction or to any prior collisional orogeny, as was suggested by some earlier studies, but instead to be basically anorogenic and reflecting continental break-up. Besides volumetrically subordinate volcanics that evolved from mantle-derived basalts (of both enriched and depleted types), abundant peraluminous orthogneisses emplaced, at least in part, as shallow intrusions, demonstrate that copious amounts of S-type granitic magmas were generated during the same event. These hot, mobile magmas, showing some geochemical resemblance with “volcanic arc” and/or “syncollision” granitoids, were produced by partial melting in the lower crust. Based on geochemical features and U-Pb age patterns of inherited zircons, it is inferred that the major source materials involved were metasediments, broadly similar to outcropping Neoproterozoic graywackes. These protoliths contained variable proportions of ancient (2 Ga and older), mature, recycled components and geochemically less mature components with a recent (ca. 540 Ma), more juvenile input. The high-temperature dehydration melting process was triggered by the advection of mantle heat, as allowed by the context of continental lithosphere extension and documented by broadly coeval basaltic magmatism on the scale of the igneous province. The large volumes of felsic magmas produced are interpreted to mirror the abundance of very fertile lithologies, such as metagraywackes, in the melting domain. In this scenario, following a proposal of Anderson and Bender (1989) for an older example of anorogenic granite magmatism, the large melt productivity would basically reflect the relatively juvenile and still largely undifferentiated nature of the local crustal segment accreted during the Cadomian orogeny.
APPENDIX: ANALYTICAL METHODS
New Major and Trace Element Data for Křivoklát-Rokycany Volcanic Rocks
The major and trace element data were obtained at the Centre de Recherche Pétrographique et Géochimique, Nancy, France, by inductively coupled plasma (ICP) atomic emission spectrometry and ICP mass spectrometry, respectively, following methods described by Carignan et al. (2001).
New Sm-Nd data for the Orlica-Śnieżnik Massif Orthogneisses
Sm-Nd isotope analyses were made in Clermont-Ferrand following isotope dilution, separation chemistry, and thermal ionization mass spectrometry methods described by Pin and Santos Zalduegui (1997). The precision of 143Nd/144Nd ratios is based on within-run statistics and quoted as the standard error on the mean at the 95% confidence level (2 standard errors). During the analyses, the average results and corresponding standard deviations (SD) obtained on Nd isotopic reference materials were m = 0.511966, SD = 0.000015 on eight measurements for the AMES R French standard, and m = 0.512114, SD = 0.000005 for six determinations of the JNdi-1 Japanese standard, equivalent to 143Nd/144Nd = 0.511857 for the previously widely used La Jolla isotopic standard (Tanaka et al., 2000).
We gratefully acknowledge the perceptive and constructive comments of the reviewers, Dr. Fritz Finger and Dr. Peter Floyd. The article is based, in large part, on results of research carried out under the long-term bilateral cooperation between Département de Géologie, Centre National de la Recherche Scientifique, Université Blaise Pascal, France, and the Institute of Geological Sciences, University of Wrocław, Poland. The Barrande Project between the Czech Republic and France is also acknowledged. Maciek Kryza helped to computerize the diagrams. This article is a contribution to the International Geological Correlation Program Project 497.
Figures & Tables
The Evolution of the Rheic Ocean: From Avalonian-Cadomian Active Margin to Alleghenian-Variscan Collision
- A-type granites
- Bohemian Massif
- Cadomian Orogeny
- Central Europe
- chemical composition
- Czech Erzgebirge
- Czech Republic
- Czech Sudeten Mountains
- felsic composition
- igneous rocks
- isotope ratios
- lower crust
- metamorphic rocks
- mineral composition
- plutonic rocks
- rare earths
- stable isotopes
- Sudeten Mountains
- Upper Cambrian
- volcanic rocks
- Kaczawa Complex
- Izera-Karkonosze Massif
- Orlica-Snieznik Massif
- Gory Sowie Massif
- Vesser Czech Republic
- Krivoklat-Rokycany Complex