Petrology, Geochemistry and Zircon Age for Redwitzite at Abertamy, NW
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ARTICLE IN PRESS Chemie der Erde 67 (2007) 151–174 www.elsevier.de/chemer Petrology, geochemistry and zircon age for redwitzite at Abertamy, NW Bohemian Massif (Czech Republic): tracing the mantle component in Late Variscan intrusions Pavla Kova´ rˇı´ kova´ a,Ã, Wolfgang Siebelb, Emil Jelı´ nekc, Miroslav Sˇtemprokc, Va´ clav Kachlı´ kc, Frantisˇek V. Holubc, Vratislav Blechac aDepartment of Mineralogy, Geochemistry and Natural Resources, Faculty of Science, Charles University, Prague, Albertov 6, 128 43 Prague, Czech Republic bInstitut fu¨r Geowissenschaften AK Mineralogie und Geodynamik, Eberhard-Karls-Universita¨t Tu¨bingen, Wilhelmstr. 56, 72074 Tu¨bingen, Germany cFaculty of Science, Charles University, Prague, Albertov 6, 128 43 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 SiO2, TiO2,Al2O3, FeO, CaO, Na2O, and K2O 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 Krusˇne´ 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 ÃCorresponding author. Tel.: +420 221 951 516; fax: +420 221 951 496. E-mail address: [email protected] (P. Kova´ rˇı´ kova´ ). 0009-2819/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2007.04.002 ARTICLE IN PRESS 152 P. Kova´ rˇı´ kova´ et al. / Chemie der Erde 67 (2007) 151–174 1. Introduction the Variscan Bohemian massif, after the intrusion of early syntectonic Variscan gabbros and associated The goal of the present study is to elucidate the role of quartz diorites of the Central Bohemian Plutonic mafic magmas in the origin of the Variscan Krusˇne´ complex (370–350 Ma, Kosˇler et al., 1993; Janousˇek hory/Erzgebirge granite batholith in the Bohemian et al., 2000) and with exhumation related ultra- massif. The presence of mafic magmas during the potassic and high K-rocks of durbachitic series of the formation of the batholith is indicated by the rare Moldanubicum (340–330 Ma, Holub, 1997; Gerdes occurrences of redwitzites associated with the granites of et al., 2000). the earlier suite in the Western pluton. The present Various geochronological methods have been used to paper contributes to the petrology and geochemistry of determine their age. Troll (1968) interpreted them as these rocks using modern laboratory methods and new basic precursors of the Fichtelgebirge granites G1–G4 genetical interpretations. on the basis of geological criteria. Taubald (2000) Redwitzites were originally defined in NE Bavaria at considered redwitzites as Upper Carboniferous in age the NW margin of the Bohemian massif by Willmann and regarded them to be younger than the early (1920). They comprise coarse- to fine-grained, porphyri- Variscan subduction-related processes at 380 Ma. The tic to equigranular igneous rocks of mafic to inter- Pb single grain evaporation method on zircons brought mediate chemical compositions ranging from quartz new data (Siebel et al., 2003) which showed an age gabbro to quartz monzonites. Their structures are highly interval between 324 and 322 Ma for the redwitzite variable, ranging from massive undeformed randomly formation. This interval is comparable with the ages oriented structures up to varieties with well developed derived by U–Pb titanite geochronology (325–322 Ma; planar structures marked by preferred orientation of Siebel et al., 2003). The zircons proved to be primary biotite flakes. minerals crystallizing from the magma not containing Their geochemistry and isotopic composition has lead from older cores as was also shown by cathodo- been studied by Troll (1968), Miessler and Propach luminescence investigation (Siebel et al., 2003). (1987), Spiegel and Propach (1991), Holl et al. The granitoids of the so-called ‘‘basic zone’’ of the (1989), Siebel (1994), Siebel et al. (1995, 2003) and Rene´ Bor massif between Tachov and Plana´ by Mar. La´ zneˇ (2000). (Vejnar et al., 1969) in the Western Bohemia were Typical dark minerals are amphibole (mostly second- studied by Siebel et al. (1997) and Rene´ (2000). These ary uralite replacing pyroxene) and biotite (both rich in rocks bear typical features identical with the redwitzites Mg but markedly poor in Al). Plagioclase (An70–An30) from NE Bavaria in terms of composition and textures is the predominant feldspar. Less common are quartz, (Siebel et al., 1997). K-feldspar, pyroxene, titanite and apatite as well as Mafic igneous rocks in the Late Variscan Krusˇne´ postmagmatic Ca–Al minerals like uralite and chlorite hory/Erzgebirge granite batholith are rare. Their occur- (Freiberger et al., 2001). In comparison with the rences have been reported from the southern part associated granites they have lower SiO2 (53–63 wt%), (Fig. 1) on the eroded slope of the Krusˇne´ hory/ lower A/CNK (o1.05), high TiO2 (0.8–1.7 wt%) and Erzgebirge mountain range (Zoubek, 1948, 1951), and low Rb/Ba (o0.18; Siebel et al., 2003). Their isotopic namely in the Czech part of the Nejdek-Eibenstock ratios indicate heterogenous enriched mantle as possible massif at Abertamy (Sattran, 1961; Sˇtemprok, 1986). 87 86 source material (eNd325 Ma ¼ +1 to À4 and Sr/ Sr325 They also occur in the Slavkovsky´ les area near Lobzy Ma ¼ 0.706–0.780; Siebel et al., 2003) modified by a (Fiala, 1961, 1968) south of the Krusˇne´ hory/Erzgebirge various degree of AFC processes and mixing with fault zone. The mafic rocks were grouped together with crustal derived magmas. the earlier group of granitoids (OIC group) as Late Redwidzites occur as small, sometimes zonal intru- Variscan granitoids. Fiala (1968) used the name red- sions, hundreds of square meters to several square witzite only for a group of plutonic rocks with planar kilometers of outcrop size, several square meters up to texture and identified other mafic rocks as biotite- several tens of meter thick dikes and tabular bodies amphibole pyroxene gabbrodiorite, biotite-amphibole intruding into Moldanubian gneisses, Saxothuringian diorite and quartz diorite to granodiorite and proposed metasediments, metabasites of the Maria´ nske´ La´ zneˇ three possible explanations of their genesis: (a) hybrid, Complex and Kladska´ unit or larger irregular bodies strongly metasomatic altered rocks of the Precambrian enclosed in host biotite porphyritic granites (Leuchten- spilitic magmatism, (b) earlier mafic intrusions of the berg, Western Krusˇne´ hory/Erzgebirge and Bor Assynthian cycle reworked by later granites, (c) earlier plutons). magmatic intrusions of the Variscan orogenic cycle. At The formation of redwidzites of NW Bohemia, the present time the third explanation is accepted Oberphalz and Fichtelgebirte represents specific pulse (Jelı´ nek et al., 2003, 2004) also for the Slavkovsky´ (340–320 Ma, Do¨ rr et al., 1998; Siebel et al., 2003; les redwitzite rock group on the basis of detailed Kova´ rˇı´ kova´ et al., 2005) of mantle derived magmas in geochemical work. ARTICLE IN PRESS P. Kova´ rˇı´ kova´ et al. / Chemie der Erde 67 (2007) 151–174 153 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 following processes of crustal relaxations. This is why the host rocks include a large variety of lithologies. The studied redwitzites occur in the Krusˇne´ hory/ The batholith emplacement is associated with the late Erzgebirge granite batholith area located in the Sax- Variscan collisional