ARTICLE IN PRESS. Pavla Kovárˇı kova. Václav Kachlı k c, Frantisˇek V. Holub c, Vratislav Blecha c. Abstract - PDF

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Chemie der Erde 67 (2007) Petrology, geochemistry and zircon age for redwitzite at Abertamy, NW Bohemian Massif (Czech Republic): tracing the mantle component in Late Variscan

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Chemie der Erde 67 (2007) Petrology, geochemistry and zircon age for redwitzite at Abertamy, NW Bohemian Massif (Czech Republic): tracing the mantle component in Late Variscan intrusions Pavla Kovárˇı kova a,, Wolfgang Siebel b, Emil Jelínek c, Miroslav Sˇtemprok c, Václav Kachlı k c, Frantisˇek V. Holub c, Vratislav Blecha c a Department of Mineralogy, Geochemistry and Natural Resources, Faculty of Science, Charles University, Prague, Albertov 6, Prague, Czech Republic b Institut für Geowissenschaften AK Mineralogie und Geodynamik, Eberhard-Karls-Universität Tübingen, Wilhelmstr. 56, Tübingen, Germany c Faculty of Science, Charles University, Prague, Albertov 6, Prague, Czech Republic Abstract A small body of mafic texturally and compositionally varied igneous intrusive rocks corresponding to redwitzites occurs at Abertamy in the Western pluton of the Krusˇne hory/erzgebirge granite batholith (Czech Republic). It is enclosed by porphyritic biotite granite of the older intrusive suite in the southern contact zone of the Nejdek- Eibenstock granite massif. We examined the petrology and geochemistry of the rocks and compared the data with those on redwitzites described from NE Bavaria and Western Bohemia. The redwitzites from Abertamy are coarse- to medium-grained rocks with massive textures and abundant up to 2 cm large randomly oriented biotite phenocrysts overgrowing the groundmass. They are high in MgO, Cr and Ni but have lower Rb and Li contents than the redwitzites in NE Bavaria. Compositional linear trends from redwitzites to granites at Abertamy indicate crystal fractionation and magma mixing in a magma chamber as possible mechanisms of magma differentiation. Plots of MgO versus SiO 2, TiO 2,Al 2 O 3, FeO, CaO, Na 2 O, and K 2 O indicate mainly plagioclase and orthopyroxene fractionation as viable mechanisms for in situ differentiation of the redwitzites. The porphyritic biotite monzogranite enclosing the redwitzite is the typical member of the early granitic suite (Older Intrusive Complex, OIC ) with strongly developed transitional I/S-type features. The ages of zircons obtained by the single zircon Pb-evaporation method suggest that the redwitzites and granites at Abertamy originated during the same magmatic period of the Variscan plutonism at about 322 Ma. The granitic melts have been so far mainly interpreted to be formed by heat supply from a thickened crust or decompression melting accompanying exhumation and uplift of overthickened crust in the Krusˇne hory/erzgebirge due to a previous collisional event at ca. 340 Ma. The presence of mafic bodies in the Western pluton of the Krusˇné hory/erzgebirge batholith confirms a more significant role of mantle-derived mafic magmas in heating of the sources of granitic melts than previously considered. r 2007 Elsevier GmbH. All rights reserved. Keywords: Fractional crystallization; Igneous petrography; I/S-type granite; Krusˇne hory/erzgebirge; Mafic rocks; Magma mixing; Pb-evaporation age; Redwitzite; Variscan magmatism; Zircon Corresponding author. Tel.: ; fax: address: (P. Kovárˇı ková) /$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi: /j.chemer 152 P. Kovárˇı ková et al. / Chemie der Erde 67 (2007) Introduction The goal of the present study is to elucidate the role of mafic magmas in the origin of the Variscan Krusˇne hory/erzgebirge granite batholith in the Bohemian massif. The presence of mafic magmas during the formation of the batholith is indicated by the rare occurrences of redwitzites associated with the granites of the earlier suite in the Western pluton. The present paper contributes to the petrology and geochemistry of these rocks using modern laboratory methods and new genetical interpretations. Redwitzites were originally defined in NE Bavaria at the NW margin of the Bohemian massif by Willmann (1920). They comprise coarse- to fine-grained, porphyritic to equigranular igneous rocks of mafic to intermediate chemical compositions ranging from quartz gabbro to quartz monzonites. Their structures are highly variable, ranging from massive undeformed randomly oriented structures up to varieties with well developed planar structures marked by preferred orientation of biotite flakes. Their geochemistry and isotopic composition has been studied by Troll (1968), Miessler and Propach (1987), Spiegel and Propach (1991), Holl et al. (1989), Siebel (1994), Siebel et al. (1995, 2003) and Rene (2000). Typical dark minerals are amphibole (mostly secondary uralite replacing pyroxene) and biotite (both rich in Mg but markedly poor in Al). Plagioclase (An 70 An 30 ) is the predominant feldspar. Less common are quartz, K-feldspar, pyroxene, titanite and apatite as well as postmagmatic Ca Al minerals like uralite and chlorite (Freiberger et al., 2001). In comparison with the associated granites they have lower SiO 2 (53 63 wt%), lower A/CNK (o1.05), high TiO 2 ( wt%) and low Rb/Ba (o0.18; Siebel et al., 2003). Their isotopic ratios indicate heterogenous enriched mantle as possible source material (end 325 Ma ¼ +1 to 4 and 87 Sr/ 86 Sr 325 Ma ¼ ; Siebel et al., 2003) modified by a various degree of AFC processes and mixing with crustal derived magmas. Redwidzites occur as small, sometimes zonal intrusions, hundreds of square meters to several square kilometers of outcrop size, several square meters up to several tens of meter thick dikes and tabular bodies intruding into Moldanubian gneisses, Saxothuringian metasediments, metabasites of the Maria nské La zně Complex and Kladska unit or larger irregular bodies enclosed in host biotite porphyritic granites (Leuchtenberg, Western Krusˇne hory/erzgebirge and Bor plutons). The formation of redwidzites of NW Bohemia, Oberphalz and Fichtelgebirte represents specific pulse ( Ma, Do rr et al., 1998; Siebel et al., 2003; Kova rˇı kova et al., 2005) of mantle derived magmas in the Variscan Bohemian massif, after the intrusion of early syntectonic Variscan gabbros and associated quartz diorites of the Central Bohemian Plutonic complex ( Ma, Kosˇler et al., 1993; Janousˇek et al., 2000) and with exhumation related ultrapotassic and high K-rocks of durbachitic series of the Moldanubicum ( Ma, Holub, 1997; Gerdes et al., 2000). Various geochronological methods have been used to determine their age. Troll (1968) interpreted them as basic precursors of the Fichtelgebirge granites G1 G4 on the basis of geological criteria. Taubald (2000) considered redwitzites as Upper Carboniferous in age and regarded them to be younger than the early Variscan subduction-related processes at 380 Ma. The Pb single grain evaporation method on zircons brought new data (Siebel et al., 2003) which showed an age interval between 324 and 322 Ma for the redwitzite formation. This interval is comparable with the ages derived by U Pb titanite geochronology ( Ma; Siebel et al., 2003). The zircons proved to be primary minerals crystallizing from the magma not containing lead from older cores as was also shown by cathodoluminescence investigation (Siebel et al., 2003). The granitoids of the so-called basic zone of the Bor massif between Tachov and Plana by Mar. La zneˇ (Vejnar et al., 1969) in the Western Bohemia were studied by Siebel et al. (1997) and Rene (2000). These rocks bear typical features identical with the redwitzites from NE Bavaria in terms of composition and textures (Siebel et al., 1997). Mafic igneous rocks in the Late Variscan Krusˇne hory/erzgebirge granite batholith are rare. Their occurrences have been reported from the southern part (Fig. 1) on the eroded slope of the Krusˇne hory/ Erzgebirge mountain range (Zoubek, 1948, 1951), and namely in the Czech part of the Nejdek-Eibenstock massif at Abertamy (Sattran, 1961; Sˇtemprok, 1986). They also occur in the Slavkovsky les area near Lobzy (Fiala, 1961, 1968) south of the Krusˇne hory/erzgebirge fault zone. The mafic rocks were grouped together with the earlier group of granitoids (OIC group) as Late Variscan granitoids. Fiala (1968) used the name redwitzite only for a group of plutonic rocks with planar texture and identified other mafic rocks as biotiteamphibole pyroxene gabbrodiorite, biotite-amphibole diorite and quartz diorite to granodiorite and proposed three possible explanations of their genesis: (a) hybrid, strongly metasomatic altered rocks of the Precambrian spilitic magmatism, (b) earlier mafic intrusions of the Assynthian cycle reworked by later granites, (c) earlier magmatic intrusions of the Variscan orogenic cycle. At the present time the third explanation is accepted (Jelı nek et al., 2003, 2004) also for the Slavkovsky les redwitzite rock group on the basis of detailed geochemical work. P. Kovárˇı ková et al. / Chemie der Erde 67 (2007) Fig. 1. Simplified geological map of NE Bavaria, southern Saxonia and NW Bohemia showing Variscan granitoid distribution. Redrawn using the geological map 1:500,000 (Czech Geol. Survey) and the data from Siebel (1993) and Rene (2000). 2. Geological setting The studied redwitzites occur in the Krusˇné hory/ Erzgebirge granite batholith area located in the Saxothuringian zone of the Bohemian massif (O Brien and Carswell, 1993). It is considered as one of the peri- Gondwana derived terranes, which formed the Armorican Terrane Assemblage (ATA; Matte, 2001; Linnemann et al., 2004). The Saxothuringian zone is interpreted as a Cadomian crustal fragment which underwent Cambro-Ordovician rifting and was affected by essentially south-eastward subduction and dextral transpression (Franke, 1989, 1993) during the (?) Silurian, Devonian and Early Carboniferous time. The south-eastern part of the Saxothuringian belt forms an antiform which exposes in its core (?) Proterozoic and Cambro-Ordovician rocks of higher metamorphic grades and in the flanks in the Northern and Western Erzgebirge/Krusˇné hory Lower Palaezoic sedimentary and magmatic rocks. The geographic location of the batholith coincides with parts of the Krusˇne hory/erzgebirge, Vogtland and Slavkovsky les. The batholith intruded as a late- to posttectonic body into various units differing in protolith ages and metamorphic evolution which were stacked together during the Variscan collision of the Tepla - Barrandian and Saxothurigian microplates (Franke, 1989; Franke et al., 2001; Kachlík, 1993, 1997; Ro tzler et al., 1998; Mingram et al., 2004) and modified during the following processes of crustal relaxations. This is why the host rocks include a large variety of lithologies. The batholith emplacement is associated with the late Variscan collisional magmatism ( Ma, Fo rster et al., 1999; Schust and Wasternack, 2002). The granites are calc-alkaline and peraluminous, evolving from early I/S- to late S-types (Sˇtemprok, 1986; Fo rster et al., 1999; Breiter et al., 1999) and form two subsequent intrusive complexes (older OIC and younger YIC, respectively, Sˇtemprok, 1986; Tischendorf et al., 1987; Tischendorf and Fo rster, 1990). The batholith is spatially divided into three plutons (Western, Central and Eastern) of which the Western pluton forms the largest outcropping body. Within this pluton the biggest intrusion is the Nejdek-Eibenstock massif (Dalmer, 1900) which is divided into Eibenstock, NW of the state boundary between Czech Republic and Germany, and Nejdek which extends south from the state boundary to the Sokolov basin. The granite bodies in the Slavkovsky les are located southeast of the Sokolov basin (Hejtman, 1984). The Czech authors so far have preferred to use the name Karlovy Vary pluton for all parts of this body (Zoubek, 1951; Sattran, 1961; Absolonova and Matoulek, 1972, 1975; Jira nek, 1982). In this paper we term the northern part of the pluton as the Nejdek-Eibenstock massif and the southern part as the Karlovy Vary massif (pluton, Fig. 1). Geological and geophysical evidence suggests that shallowly emplaced granites can be traced from the 154 P. Kovárˇı ková et al. / Chemie der Erde 67 (2007) eastern contact to a distance of approximately 15 km eastwards from the surface exposures (Tischendorf et al., 1965). Intrusions of mafic rocks are known only from the southern part of the batholith in the area NW of Karlovy Vary and in the Slavkovsky les (Karlovy Vary massif). However, numerous mafic rocks are a component of the bimodal dyke assemblage (Sˇkvor, 1975) that includes gabbros, diorites, granodiorites and lamprophyres. Recent dating of the Eibenstock part of the Nejdek- Eibenstock massif gave a 207 Pb/ 206 Pb age of 32078Ma for the megacrystic Eibenstock granite and of Ma for a rhyolitic dyke suggesting a significant time gap between the main granite intrusion and anorogenic rhyolite dykes, which intruded breccia bodies of tin-hosting greisens (Kempe et al., 2004). For the Kirchberg granite an age of Ma was defined by U Pb uraninite geochronology (Kempe, 2003). 3. Redwitzite body at Abertamy The redwitzite body at Abertamy is located in the Western Krusˇne hory/erzgebirge about 7 km NW of Ja chymov. The northern contact zone of the granitoid massif is bordered by mica schists (Fig. 2) and two-mica gneisses and intersected by abundant dykes of granite porphyries. The south-eastern part of the area is occupied by a Tertiary nephelinite which intersects the granite. Mafic intrusions are confined to the endocontact of the massif which is formed by coarse-grained and medium-grained OIC granite. The earlier geological map distinguished between these two granite varieties. However, owing to the lack of outcrops, their cartographic representation is difficult and not possible to identify reliably as separate bodies without a new trenching. The earlier mapping (Zoubek, 1947; Sˇkvor and Sattran, 1974) revealed a very irregular shape of this particular outcrop. The new mapping in scale 1:10,000 of the first author (using shallow cartographic drilling) showed that the two redwitzite outcrops can be mutually interconnected below the Quaternary cover sediments and interpreted as a single outcropping lensoid body of about km size of NE SW strike (Fig. 2). The geophysical data (see below) point to a shallow extension of the redwitzite body. The redwitzite body at Abertamy was studied by Zoubek (1948) who interpreted the rock, which he referred to as gabbrodiorite, to be younger than the OIC granites based on the surface shape resembling a lobate dyke. In general schemes, Zoubek (1951) and Sˇtemprok (1986, 1992) considered, however, the redwitzites as the earliest members of Variscan granitoid magmatism. In the south-western part of the redwitzite body we identified a thin dyke of biotite granite with a sharp contact against the redwitzite. This suggests that at least some contacts between the redwitzite and the granite are intrusive and the OIC granite intruded later than the redwitzite. A complete silicate analysis for the Abertamy gabbrodiorite is given by Sattran (1963) compared with some other igneous and metamorphic rocks of the Krusˇne hory/erzgebirge. A more detailed petrological and petrochemical description of gabbrodiorites from the western part of the Krusˇne hory/erzgebirge pluton is published in Sˇtemprok (1986) and of the Abertamy gabbrodiorite in Sˇtemprok (1992). The results of a new study of redwitzites are reported in Jelı nek et al. (2003, 2004). 4. Sampling Samples (5 15 kg) for petrological and geochemical studies were taken from large boulders, from a section in the valley of Brook Bystrˇice and from boulders in the Fig. 2. Geological map of the Abertamy mafic intrusion with the profiles of geophysical measurements. P. Kovárˇı ková et al. / Chemie der Erde 67 (2007) river base. The samples were collected in two campaigns: one in (Sˇtemprok, 1992, samples labelled as PLE) and the second in 2002 (Kova rˇı kova, 2004, samples labelled as AB). The list of samples is given in Jelı nek et al. (2003). 5. Analytical techniques A Scintrex CG-3M gravimeter was used for gravity survey and a proton magnetometer PM-2 was used for measurement of total component of magnetic field. The mean square error calculated from repeated measurements is mgal for gravity and 72 nt for magnetic values. Gravity measurements were processed to the form of relative Bouguer anomalies Dg with complete topographical corrections (radius km from measured stations). DT values were calculated from total field magnetic measurements. The normal magnetic field was set as a median of the measured values at each locality. Major and trace element analyses on selected wholerock samples have been conducted in the Chemical laboratory of the Czech Geological Survey in Prague by conventional wet methods (silicate analyses, major elements), by optical emission spectrometry (OES) (B, Be, Bi, Co, Cr, Cu, Ga, Mo, Pb, Zn), by X-ray fluorescence analyses (XRF) (Nb, Y, Zr, U), inductively coupled plasma mass spectrometry (ICP-MS) (REE and Y, Sc, Th, Ta, W) and atomic absorbtion spectrometry (AAS) (Sr, Ba, Cs, Rb, Zn, V and Ni) methods. Additional analyses have been performed in the chemical laboratory of the Faculty of Science, Charles University, Prague (wet silicate analyses and AAS for Li, Rb, Cu, Co, Cr, Ni, Zn, Sr, Pb and ICP-MS for REE and Ba, Hf, Sc, Nb, Ta, Th, U). The minerals were analyzed by electron microprobe in the Chemical laboratory of the Czech Geological Survey in Prague (CamScan S-4 Link ISIS 300), at the Faculty of Science, Charles University, Prague (CamScan S-4 Link ISIS EDX) and at the Laboratory of the Institute of Geology of the Academy of Science (CAMECA SX- 100 electron microprobe in the wavelength dispersive mode). The CamScan microprobe was used for the backscattered electron images of zircons. Heavy mineral fractions were prepared in the Laboratories of the Czech Geological Survey in Barrandov, Prague. The crushed samples were separated in heavy liquids and zircons were handpicked from the heavy mineral assemblage after magnetic separation. For single-zircon Pb-evaporation (Kober, 1986, 1987) chemically untreated zircon grains were analyzed with a Finnigan MAT 262 mass spectrometer equipped with a secondary electron multiplier (SEV) at the University of Tübingen. Principles of the evaporation method used in this study are described in Siebel et al. (2003). Withthe exception of four grains from sample AB4, only data with high radiogenic Pb component ( 204 Pb/ 206 Pbo ) were used for evaluation. One grain from sample AB7 yielded very high Pb intensities and masses 206, 207 and 208 were also detected simultaneously in Faraday cups. All 207 Pb/ 206 Pb ratios were corrected for common Pb according to the formula given in Cocherie et al. (1992) following the two stage growth model for the evolution of Pb isotopic ratios of Stacey and Kramers (1975). No correction was made for mass fractionation. The common Pb corrected 207 Pb/ 206 Pb ratios normally define a Gaussian distribution and the mean of the 207 Pb/ 206 Pb ratios was derived from this distribution. The error for a single zircon age was calculated according to the formula sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D age ¼ p 2s 2 ffiffi þ Df 2, n where n is the number of 207 Pb/ 206 Pb isotope ratio scans, 2s is the 2sigma standard error of the Gaussian distribution function and Df an assumed error for the measured 207 Pb/ 206 Pb ratios of 0.1% which includes potential bias caused by mass fractionation of Pb isotopes and uncertainty in linearity of the multiplier signal. The mean age for one zircon from sample AB7 is given as weighted average and the error refers to the 95% confidence level (ISOPLOT, Ludwig, 1999). Repeated measurements on two internal standard zircons of similar age show that most of the investigated samples were performed for geologically realistic age and error treatment. 6. Petrography 6.1. Granites The biotite granite in the surrounding of Abertamy is medium- to coarse-grained, and has
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