The phonolites were studied from nine representative localities in the Lužické hory Mts. area, which are mainly associated with the Lusatian Fault. The locations of the sampling sites are shown in Fig. 1 and the geographical coordinates are given in Table 1.
The phonolite samples are greenish-grey up to dark grey in colour, fine-grained to aphanitic, sometimes (micro)porphyritic and vesicular rocks. Only samples from Tolštejn and Sokol near Petrovice with light greenish colour display macroscopic evidence for some low-temperature alteration. Fine-grained, rarely glassy trachyte-textured groundmass sometimes shows signs of flow structure. The groundmass consists of abundant alkali feldspar with interstitial plagioclase, feldspathoids, and rare mafic minerals. The phenocrysts are feldspars, feldspathoids of the nepheline and sodalite group, and microphenocrysts of clinopyroxene prevailing over amphibole, titanite and apatite. Both cognate and true xenoliths are rare.
Alkali feldspar, anorthoclase and Na-sanidine are most abundant, partly with elevated contents of the An-component (3–5 mol%; Kühn 1990). They occur in the groundmass and as phenocrysts. Feldspars laths of the groundmass are often fluidal arranged (e.g., Klíč). Sodalite-rich phonolites contain predominantly Na-sanidine and anorthoclase; K-sanidine is very rare. Alkali feldspars contain cores higher in Ba (Cn-component = 2–4 mol%; Kühn 1990).
Plagioclase occurs mostly in the groundmass and rarely as phenocrysts in partly trachytic samples from Polevský vrch and Velký Buk only. It is present only in some sodalite-rich phonolites represented prevalently by K-oligoclase with increasing Cn-component (up to 4 mol%; Kühn 1990).
Feldspathoids (unpublished microprobe data of JU) occur in various proportions as groundmass and rarely phenocryst phases in all phonolites. Nepheline was partly converted to analcime or to a mixture of clay minerals and zeolites. Nepheline phenocrysts are partly resorbed, and occur as relics (Ks-component of 10–15 mol%). Relicts of nepheline phenocrysts correspond mostly to Si-poor type. Nepheline grains in groundmass tend to be the Si-rich types formed from the highly fractionated melts during the final stages of crystallization (Dollase and Thomas 1978). Both types of nepheline correspond to ideal composition of natural nepheline sensu Dollase and Thomas (1978). Sodalite is abundant in the sodalite-bearing phonolites of Jedlová, Klíč, Luž and Velký Buk (characterized by high Cl contents). Haüyne occurs rarely in cores with sodalitic grains or as discrete euheral microphenocrysts with calcite rims. Analcime is present not only as pseudomorphs after nepheline but as an independent interstitial phase. Glass is rare in the groundmass associated not only with rapid magma crystallization, but partly also to the low-temperature alteration products of felsic minerals of groundmass.
Clinopyroxen e occurs either as euhedral to subhedral phenocrysts often of prismatic shape or as anhedral to subhedral fine-grained uneven grains, needless and aggregates elongated parallel to the fluidal structure of groundmass. Characteristic Al and Ti variations in the composition of phenocrysts correspond to concentric zoning. Their contents generally decrease from core to rim and groundmass grains. Concentric, rarely oscillatory zoned phenocrysts have usually rare cores of diopside composition, and prevailing hedenbergite grading to aegirine-augite composition (classification of Morimoto et al. 1988) and pure aegirine in the rims (Kühn 1990 and unpublished data of JU). Aegirine is in particular present as needles in the groundmass. Contents of ZrO2 in hedenbergites are 0.5–0.7 wt%, while they reach up to 1.2 wt% in aegirine-augites and 5.7 wt% in aegirines (Tolštejn, Sokol near Petrovice and Klíč; Kühn 1990 and unpublished data of JU).
Amphiboles of ferroan pargasite to kaersutite composition (classification of Leake et al. 1997) occur rarely as phenocrysts and/or xenocrysts (Kühn 1990; unpublished data of JU). Skeletal development is often characteristic (e.g., Tolštejn). Magnesian riebeckite is present in phonolite from Klíč. Rare hornblendite cognate xenoliths of 10 to 20 mm in size consist of ferroan pargasite to kaersutite ± diopside, apatite and titanite. They often disaggregated to amphibole xenocrysts of 1–12 mm in length.
Titanian magnetite is present in groundmass or as small subhedral microphenocrysts in all phonolite samples. It belongs to the haematite–ilmenite solid solutions series showing variations in the TiO2 content (5–10 wt%; Kühn 1990).
Fluorapatite and Nb-rich titanite belong to the common accessories of the phonolites and form often clusters with titanian magnetite and aegirine-augite. Zirconian silicate hainite was originally described from Chlum Hill (Hoher Hain) by Blumrich (1883). It occurs in the groundmass and as needles in miarolitic cavities of zirconian-rich phonolites, e.g., from Klíč, Tolštejn or Sokol near Petrovice (Skála et al. 2010; Ulrych et al. 1992). Kühn (1990) presented rims of zirconian silicate and oxide vlasovite around hainite tabular crystals from the Luž phonolite. High fluorine contents (5.6 to 9.1 wt%) together with impregnations of fluorite around the fluorian eudialyte in the groundmass of zirconian-richest phonolite (2650 ppm) from Sokol near Petrovice support its metasomatic origin (Ulrych et al. 1992).
Major and trace-element geochemical characteristics
Major and trace-element analyses of the studied phonolites from the Lužické hory Mts. are given in Table 1. Two geochemically different phonolite types were recognized with respect to Sr and Ba contents – type A (high Sr and Ba) and type B (low Sr and Ba).
The rocks plot mainly into the phonolite field in the TAS (Total Alkali-Silica; Le Maitre 2002) diagram (Fig. 2). The type A phonolites compositionally overlap with the majority of phonolites from the České středohoří Mts. (CS; Ackerman et al. 2015), while the type B phonolites are characterized by more scattered distribution in the TAS diagram (Fig. 2). Most of the phonolites are peralkaline with the values of the alkalinity index [molar (K2O + Na2O)/Al2O3] ~ 1. All samples have low Mg# [molar 100 × MgO/(MgO + FeOtot)] ranging from 3 to 15, except for the phonolite from Sokol near Petrovice (sample NBPH3) yielding much higher Mg# of 69, most likely due to incorporation of some mantle-derived xenocrysts.
The major elements of the phonolites from the Lužické hory Mts. display only slight correlation among main oxides. Generally, TiO2 contents correlate positively, whereas Al2O3 and Na2O are predominantly negatively correlated with increasing SiO2 contents. The variation diagrams of selected major elements indicate partial compositional similarity of the type A phonolites with those from the CS and wide variation of the type B samples (Fig. 3).
The presence of single differentiation series in the phonolites studied is indicated in the trace element variation diagrams such as Zr vs. Rb or Th vs. U (Fig. 4). The presence of two series are best displayed in Rb vs. Sr, Nb vs. Zr, U/Pb vs. Zr/Nb and Rb/Sr vs. Nb/Ta plots (Fig. 4).
The geochemical behaviour of major and trace elements during magmatic differentiation within the investigated phonolite types can be best seen in the normalized trace-element patterns (Fig. 5). The trace-element characteristics for most of the type A phonolites from the Lužické hory Mts. are similar to those described from the CS by Ackerman et al. (2015). Investigated type B phonolites (Velký Buk, Tolštejn, Sokol near Petrovice) were more affected by late-magmatic and post-magmatic processes compared to phonolites from the CS, which is confirmed by distribution of the primitive mantle normalized trace-element pattern with pronounced Rb, Th, U, Nb, Pb and Zr peaks, along Ba, Sr and Ti troughs (Fig. 5).
Chondrite-normalized rare earth element (REE) patterns of the studied phonolites partly overlap, suggesting similar partial melting and fractional crystallization histories (Fig. 6). The samples show minor signs of U-shaped patterns with low abundances of the MREE. Negative Eu anomalies (Eu/Eu* = 0.4–0.6, higher Gd/Gd* = 1.2–1.7 and lower LaN/YbN = 10–29) are characteristic for B type of phonolites, while the A types of phonolites are characterized in the transitional values of Eu/Eu* = 0.6–1.1, lower Gd/Gd* = 0.28–1.6 and higher LaN/YbN = 29–34 (Table 1). Compared to the CS samples, REE patterns of the studied phonolites are more scattered (Fig. 6). While the phonolite samples from Velký Buk and Sokol near Petrovice (type B) show the highest LREE enrichment, the sample from Luž (type B) displays a marked depletion of all REEs.
Two phonolite whole-rock samples from the Lužické hory Mts. were analyzed for K–Ar ages (Table 2). The phonolites show ages of 34.2 ± 1.4 and 26.7 ± 1.1 Ma corresponding to the age of phonolites from the CS (~ 36–23 Ma; Ackerman et al. 2015) and the genetically associated basaltic rocks of the CS (36–21 Ma; Ulrych et al. 2002). However, we note that the ubiquitous excess of argon supposed to be preferentially concentrated in nepheline and/or sodalite (e.g., nepheline-rich and sodalite-poor phonolite from Sokol near Petrovice – 34 Ma) could largely account for the spread in the K–Ar ages (Balogh et al. 1999). Balogh in Ulrych et al. (2000b, 2005) and Pivec et al. (2004) assumed that a submicroscopic alteration associated with the change of the mineral structure may be responsible for the lowered Ar retention of nepheline and leucite and lower age.
Sr–Nd–Pb isotope characteristics
The initial 87Sr/86Sr and 143Nd/144Nd data of phonolites from the Lužické hory Mts. show a very large spread in Sr isotope ratios from ~ 0.6979 to 0.7142 contrasted by a relatively narrow range of εNd values from + 3.0 to + 3.9 (Fig. 7; Table 3). In particular, the type B phonolites are characterized by contrastingly variable initial 87Sr/86Sr ratios, showing a decoupled horizontal trend on the Sr–Nd isotope diagram (Fig. 7). Such a large variation in Sr isotopes at fairly constant εNd values documents that the Rb-Sr isotope system in these rocks was severely disturbed during rock transformation modifying the Rb/Sr ratios and the 87Sr/86Sr ratios.
Initial lead isotope composition of both phonolite types is relatively uniform, yielding 206Pb/204Pb = 19.20–19.50, 207Pb/204Pb = 15.61–15.66 and 208Pb/204Pb = 39.07–39.30 (Fig. 8; Table 3). All samples form a nonlinear, rather scattered array with more distinct variation in 207Pb/204Pb isotope ratios in the 207Pb/204Pb vs. 206Pb/204Pb diagram. In contrast, the analyses show a nearly linear trend with only minor fluctuation of 208Pb/204Pb ratios at given 206Pb/204Pb values in the 208Pb/204Pb vs. 206Pb/204Pb diagram (Fig. 8).