The epidote-group minerals (Armbruster et al. 2006) are locally important carriers of Sr due to a common substitution of Sr2+ for Ca2+. In igneous and metamorphic rocks, total concentrations of Sr in epidote minerals depend on multiple factors, mainly on bulk Sr content, P-T conditions, fluid-rock interaction and presence of other potential Sr-carriers, e.g., feldspars, apatite, calcite, dolomite, aragonite, strontianite, witherite, baryte, celestine and others (Davidson 1998; Frei et al. 2004). Epidote-group minerals rich in Sr were mostly reported from HP-UHP metamorphic rocks, especially amphibolized eclogites, jadeitites and tectonized serpentinites (e.g., Brastad 1985; Mottana 1986; Harlow 1994; Nagasaki and Enami 1998), manganese deposits (Bonazzi et al. 1990; Armbruster et al. 2002; Minakawa et al. 2008; Cotterell and Tayler 2012; Tanaka and Hamane 2016), and some specific lithologies, such as metagreywacke-quartzofeldspathic schists metamorphosed under prehnite-pumpellyite to pumpellyite-actinolite facies (Grapes and Watanabe 1984), prehnite-rich rodingites derived from serpentinites (Miyajima et al. 2003), albitite dykes in serpentinized lherzolites (Monchoux et al. 2006), or hydrothermally altered peralkaline alkali-feldspar granites (Vlach 2012).
Two Sr-dominant members of the epidote-group minerals (Armbruster et al. 2006) were approved by CNMNC IMA: piemontite-(Sr) (Bonazzi et al. 1990) and epidote-(Sr) (Minakawa et al. 2008). The epidote-(Sr) [CaSrAl2Fe3+(Si2O7)(SiO4)(OH)] was firstly reported by Minakawa et al. (2008) from hydrothermal veins in a tinzenite deposit hosted by metachert (Nagakawara deposit) and piemontite breccias (Hohnomori deposit) at the Ananai mine in Japan. Specimens from these two occurrences contain 17.1–18.0 and 10.5–16.4 wt. % SrO, respectively, and are often enriched in piemontite-(Sr) component. Cotterell and Tayler (2012) identified epidote-(Sr) and piemontite-(Sr) associated with pyrophanite in celsian from the dumps of the Fe-Mn Benallt mine (Llŷn Peninsula, United Kingdom), and Tanaka and Hamane (2016) briefly described a third occurrence of epidote-(Sr) from Shiromaru mine (low-grade Mn deposit), Japan, which forms hydrothermal veins in hematite-rich metachert.
In this work, we deal with new occurrence of epidote-(Sr) and Sr-rich epidote related to hydrothermally altered alkaline igneous rocks of the teschenite association in the Silesian Unit (Outer Western Carpathians, Czech Republic). We present data on the paragenesis, chemical composition of the Sr-rich epidote-group minerals and whole-rock Sr-isotope data and try to clarify the genesis of epidote-(Sr) and the origin of Sr for its formation.
The investigated Sr-rich members of the epidote-group occur in leucocratic dykes penetrating mafic host rocks. Both belong to the teschenite association, which is defined as a heterogenous suite of mostly alkaline magmatic rocks geochemically equivalent to alkaline basalts, basanites, nephelinites and picrites. The rocks of the teschenite association occur in the area between the towns of Hranice in Czech Republic and Bielsko-Biała in Poland (e.g., Pacák 1926; Smulikowski 1930; Šmíd 1978; Kudělásková 1987; Hovorka and Spišiak 1988; Włodyka and Karwowski 2004). This area belongs to the Silesian Unit of the Flysch Belt within the Outer Western Carpathians. The Silesian Unit is a part of the Krosno group of nappes, i.e., remnant of sedimentary basins developed on the margin of the European Platform and incorporated into the Carpathian accretion wedge during the Alpine orogenic event in the Cenozoic (Stráník et al. 1993; Plašienka 1997; Froitzheim et al. 2008). The Silesian Unit can be regarded as a remnant of an extensional basin formed during the Jurassic/Cretaceous rifting at the southern margin of the European Platform (Nemčok et al. 2001). Sedimentation in the Silesian Unit started in the Oxfordian-Berriasian (Eliáš 1970; Menčík et al. 1983) by shallow-water bioclastic to micritic Štramberk limestone and deep-water calcareous claystones of the Vendryně Fm. (Eliáš et al. 2003) with bodies of the Těšín limestone (Stráník et al. 1993). Typical rhythmic and cyclic flysch sedimentation started by deposition of the Hradiště Fm. during the Valanginian-Aptian (Eliáš et al. 2003). Rhythmic alternation of dark calcareous claystones and laminated sandstones is characteristic for the base of Hradiště Fm. whereas beds of sandstone and conglomerates with cobbles of the Štramberk limestone and overlying dark-gray weakly calcareous claystones dominate in the upper part.
Lithology of the Hradiště Fm. includes layers of dark organic silicites, pelocarbonate horizons and bodies of igneous rocks of the teschenite association, which form mostly hypabyssal sills, submarine extrusions, pillow lavas and volcanoclastics (Stráník et al. 1993). The 40K-40Ar and 39Ar-40Ar whole-rock dating of the teschenites reveal their Lower Cretaceous age of ~138–120 Ma (Lucińska-Anczkiewicz et al. 2002; Grabowski et al. 2003), in-situ mineral U-Pb dating gave a Lower Aptian age (~120 Ma; Szopa et al. 2014; Matýsek et al. 2018; Brunarska and Anczkiewicz 2019). The alkaline magmatism could be related with early rifting (Narebski 1990; Spišiak and Hovorka 1997; Brunarska and Anczkiewicz 2019) or with reactivation of deep faults during the Lower Cretaceous (Dostal and Owen 1998). Based on the Nd, Sr, and Hf isotopic composition and the trace element contents, the source magma was probably a product of ~2–6 % partial melting of upper mantle garnet peridotite at a depth of ~60–80 km. This magma was compositionally similar to ocean island basalts (OIB) with HIMU (high-μ; μ = 238U/204Pb) affinities, possibly modified by mixing with more depleted, MORB-type component (Dostal and Owen 1998; Harangi et al. 2003; Brunarska and Anczkiewicz 2019). The deposition of the Hradiště Fm. occurred above the carbonate compensation depth (Halásová et al. 2013). As a result of the Alpine orogenic event, the entire rock sequence was folded and thrusted towards NW on the Bohemian Massif (Stráník et al. 1993). Regional diagenetic thermal overprint of sediments of the Silesian Unit reached up to 170 °C (Botor et al. 2006), but local tectonic processes generated rarely vein mineralizations which formed at temperatures up to 220 °C (Dolníček et al. 2012; Urubek et al. 2014).
Both rocks of the teschenite association and underlying carbonate sediments of the Vendryně Fm. are known for occurrence of Sr-minerals. To date, four Sr-minerals have been identified in hydrothermally altered teschenites, namely: strontianite (Dolníček et al. 2010a), slawsonite (Matýsek and Jirásek 2016; Schuchová 2016), fluorcaphite (Kropáč et al. 2017) and epidote-(Sr) (Kropáč et al. 2020 and this paper). In addition, Sr2+ was observed in feldspars, apatite- and epidote-group minerals, zeolites (thomsonite-Ca) and carbonates from the teschenite rocks (Spišiak and Mikuš 2008; Kynický et al. 2009; Dolníček et al. 2010a; Matýsek and Jirásek 2016; Schuchová 2016; Kropáč et al. 2017, 2020). Bulk Sr contents in the teschenites vary mostly ~600–2250 ppm (average of 23 analyses attains 1250 ppm; Dolníček et al. 2010a,b; Dostal and Owen 1998; Harangi et al. 2003; Schuchová 2016; Kropáč et al. 2017; Brunarska and Anczkiewicz 2019), but, exceptionally, Sr contents can reach up to 10320 ppm (Schuchová 2016) in the studied dykes of analcime-syenitic composition. Bulk Sr contents in picrites vary only between ~250–1290 ppm (Dostal and Owen 1998; Dolníček et al. 2010b; Brunarska and Anczkiewicz 2019) and Sr is mainly incorporated in secondary K-feldspar, carbonates and baryte (Dolníček et al. 2010b; Kropáč et al. 2015; Jirásek et al. 2017). The Sr-bearing minerals in the Vendryně Fm. are strontianite, celestine and Sr-rich baryte. The Sr- and Ba-rich hydrothermal mineralization occurs in fissures of gray-black bituminous limestone beds and concretions which are enclosed in layers of calcareous dark-gray claystones (e.g., Slavíček 1985; Jedlička 1988; Marosz and Chmiel 2007; Skýpala 2014).
Occurrence and paragenesis
The Čerťák occurrence (49°33'58"N, 17°59'54"E) is situated ca. 2 km south from the town of Nový Jičín, near a same-named water reservoir. It represents one of the best exposures of a teschenite sill, which runs in SW–NE direction between Kojetín and Bludovice villages in a total length exceeding 2 km (Fig. 1). The sill is locally over 30 m thick and compositionally heterogeneous (Matýsek and Jirásek 2016; Kropáč et al. 2020). Hydrothermally altered mesocratic fine- to coarse-grained teschenites dominates. They consist mainly of clinopyroxene and amphibole phenocrysts, biotite, apatite, analcime-feldspar groundmass, and accessory or secondary titanite, magnetite, chlorite, carbonates, and pyrite. Less common are melanocratic pyroxene-rich varieties, as well as leucocratic types (M´ = 20–35) which form up to 7 cm thick fine- to medium-grained dykes or several cm big nests randomly distributed in the mesocratic teschenite (Kropáč et al. 2020).
The mineral association of leucocratic dykes (Fig. 2) from the studied site was recently described by Matýsek and Jirásek (2016) and Kropáč et al. (2020). They consist mainly of subhedral lamellae or anhedral irregular grains of alkali feldspars (albite and K-feldspar), hyalophane, celsian (£0.21 apfu Sr) and rarely slawsonite (≤0.91 apfu Sr; Matýsek and Jirásek 2016). The alkali feldspars are surrounded or corroded by analcime and natrolite, less frequently by thomsonite-Ca with up to 0.27 apfu Sr which probably replaced primary plagioclase (Kropáč et al. 2020). Mafic components are sporadically represented by euhedral prismatic phenocrysts of clinopyroxene (diopside rimmed by hedenbergite or aegirine-augite to aegirine), long-prismatic calcic amphibole (kaersutite or ferrokaersutite with hastingsite or ferropargasite rim) and platy crystals of annite. In addition, the mineral association includes Sr-free fluorapatite (F = 0.60–1.01 apfu; own unpublished data), Ti-rich magnetite, prehnite, chlorite (chamosite), pyrite, calcite, baryte, (OH, F)-rich grossular, rare HFSE-, REE-rich accessory minerals (Zr-Nb-rich titanite, pyrochlore, zircon, REE-rich fluorapatite and vesuvianite; Kropáč et al. 2020), and epidote group minerals which are characterized in detail below.