Petrographic and geochemical interpretation of the Late Cretaceous volcaniclastic deposits from the Hateg Basin

Palaeogeography, Palaeoclimatology, Palaeoecology 293 (2010) 306–318

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Petrographic and geochemical interpretation of the Late Cretaceous volcaniclastic deposits from the Hateg Basin Sorin C. Barzoi ⁎, Marin Seclaman Department of Mineralogy, Faculty of Geology and Geophysics, University of Bucharest, 1, N. Balcescu Blvd., 1, RO-010041, Romania

a r t i c l e

i n f o

Article history: Received 24 November 2008 Received in revised form 23 May 2009 Accepted 28 August 2009 Available online 10 September 2009 Keywords: South Carpathians Hateg Basin Volcanism Pyroclastic rocks Andesites

a b s t r a c t The present paper provides new petrographic and geochemical insights into the tectonic and depositional environment of the Late Cretaceous volcaniclastic deposits from the Hateg Basin, in the South Carpathians. These deposits are widespread in the Rachitova–Stei unit, which is tectonically delimitated from the South Carpathians' geological background by strike-slip faults. The conducted petrographic studies have revealed that the volcaniclastic rocks from the Rachitova–Stei unit are mainly pyroclastic (i.e. coarse tuffs, tuff-breccia and tuffites), with a wide variety of volcanic pyroclastic fragments, most commonly including andesites (amphibole andesites, biotite-bearing-amphibole andesites, pyroxene-bearing-amphibole andesites, pyroxene-andesites), and less frequently latite-andesites, latites, rhyolites and dacites. The rhyolites are found only in the lower sequence of the Rachitova–Stei unit. The volcaniclastic deposits were altered by diagenetic processes. The celadonite is the most common diagenetic mineral formed in these rocks, whose presence indicates that the pyroclastic fragments have interacted with sea water. The high percentage of pyroclastic fragments with mainly angular forms and the rich-hornblende content evidence a highly explosive volcanism. The poorly-sorted pyroclastic assemblages with larger blocks and with little reworked pyroclastic material indicate a very short distance of transportation and a deposition relatively close to the explosion centre. The huge quantity of pyroclastic material from the Rachitova–Stei unit suggests that the volcanic explosion had a centre of considerable dimensions, but there is no evidence of such a volcano neither in the Hateg Basin nor anywhere else in the South Carpathians. The whole-rock geochemical composition and the immobile trace element signature from the pyroclastic fragments indicate an island arc depositional environment that was placed close to an active continental margin. In addition, the andesitic nature of the pyroclasts indicates that the volcaniclastic rocks were most probably derived from a magma generated by the subduction of an oceanic plate under a thin continental plate. The presence of the non-volcanic pyroclasts (i.e. granites, quartz-muscovite schists, muscovite quartzites, graphitic phyllites etc.), detached from the continental crust by explosion, confirms that the overriding plate is of continental material. The presence of the rhyolitic magma in the first stage of eruption also supports the continental nature of the overriding plate, the magma having resulted by the partial melting of the rocks from the continental crust. The petrographic and geochemical data have evidenced that the volcanism took place on a thin continental crust. However, this crust is not characteristic to the Getic Unit from the South Carpathians, which is commonly accepted to represent the basement of the Hateg Basin. Most probably, the volcanism was generated in another geotectonic context, respectively, in a continental island arc with a thin overriding crust, most likely situated at a lower latitude. The volcaniclastic units of the Hateg Basin were displaced and moved from this tectonic context to the present location by strike-slip movements. © 2009 Elsevier B.V. All rights reserved.

1. Introduction This paper expresses the petrographic and geochemical characteristics of the volcaniclastic deposits from the Hateg Basin, South Carpathians; deposits considered by most previous authors (e.g.

⁎ Corresponding author. Tel.: +4 075 637 4749. E-mail address: [email protected] (S.C. Barzoi). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.08.028

Grigorescu, 1992) to represent a Maastrichtian post-tectonic fluviolacustrine continental basin filled predominantly with volcaniclastic rocks and with dinosaur-bearing rocks (Fig. 1). While the dinosaurbearing rocks gained enormous interest for study (e.g. Grigorescu, 1983; Grigorescu et al., 1985; Grigorescu and Anastasiu, 1990; Grigorescu, 1992; Grigorescu et al., 1994; Grigorescu and Csiki, 2002), the geochemical features of the volcaniclastic rocks and the petrologic processes in these deposits remained incompletely investigated. Previous petrographic studies on these volcaniclastic rocks

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Fig. 1. (a) Regional map of the Carpathian Mountains. The Hateg Basin is located in the southern part of the Carpathians. The boxed area is detailed in b; (b) Geologic map of the Hateg Basin and surrounding area (based on the authors' data and the map from Van Itterbeeck et al., 2005). The volcaniclastic deposits presented in this paper are located in the western part of the Hateg Basin, in the Rachitova–Stei unit.

(Grigorescu, 1983; Grigorescu et al., 1985; Grigorescu and Anastasiu, 1990; Grigorescu, 1992; Grigorescu et al., 1994) were performed using mainly optical microscopy; hence, the whole-rock geochemistry and the chemical composition of the minerals were not analysed. Such analyses are imperative to revealing the physico-chemical conditions of volcanism and the tectonic setting at the time of eruption. Recent research results (e.g. Wei et al., 2004) showed that the geochemical signature of volcaniclastic rocks can provide highly reasonable constraints for inferring depositional basin features. The purpose of the present work is to analyze the volcaniclastic fragments from the Hateg Basin, their petrography, mineralogy and geochemistry, and deduce (i) the nature and origin of the magma, (ii) the tectonic setting of the volcanism and (iii) the location of the explosion centre. 2. Geological background The volcaniclastic deposits presented in this paper are located in the western part of the Hateg Basin, in the South Carpathians (Fig. 1a, b), where there are separated two geological units, the Densus-Ciula Formation and the Sinpetru Formation, described together as a Late Cretaceous syn- and post-Laramide orogenic continental basin (e.g. Willingshofer et al., 2001). The general geology of the field area is shown in the map from Fig. 1b. The Densus-Ciula Formation crops in a large area in the northwestern part of the basin (Fig. 1b) and is described by previous authors as measuring 3 to 4 km in thickness. It consists of a lower member containing volcaniclastic material, a middle member with fossils, but poor in volcaniclastic material, and a non-fossiliferous upper member with no volcaniclastic material (e.g. Grigorescu, 1992). According to Anastasiu (1991), Grigorescu (1992), Grigorescu et al. (1994) and Grigorescu and Csiki (2002), the Densus-Ciula formation resulted by sedimentation of pyroclastic material mixed with epiclastic material in

a lake basin, followed by a fluvio-lacustrine sedimentation of epiclastic and pyroclastic material, reworked from the lower member. The Sinpetru formation crops in the Sibisel valley (Fig. 1b) and measures about 2 to 2.5 km in thickness, being generally subdivided into a lower member with red clays and containing slightly disseminated pyroclastic material, and an upper member with conglomerates and a lack of red clays (Grigorescu, 1983; Weishampel et al., 1991; Grigorescu, 1992). Most of the authors have characterized this formation as a fluvio-lacustrine facies generated distal from the volcanic centre (Grigorescu, 1983; Weishampel et al., 1991; Grigorescu, 1992; Grigorescu and Csiki, 2002). The field mapping completed by the authors of the present study has revealed a tectonic fragmentation of the Hateg Basin deposits, mostly as a result of several strike-slip faults. Therefore, within the Densus-Ciula Formation two distinctive tectonic units have been separated: the Rachitova–Stei unit and the Tustea unit (Fig. 1b). The former consists mainly of pyroclastic rocks, containing more than 75% modal pyroclastic fragments, while the latter consists of volcaniclastic sedimentary rocks that only sporadically contain pyroclastic fragments (less than 25% modal). That makes the Rachitova–Stei unit much more appropriate for studying the volcanism. The volcaniclastic deposits from this unit unconformably overlie the Cretaceous marine deposits, measure at least 100 m in thickness and crop largely in the Rachitova and Stei valleys. 3. Petrography of the volcaniclastic fragments More than five hundred samples of volcaniclastic material have been collected for microscopic studies. The petrographic data and the modal composition presented below are based on 207 thin sections of selected samples from the Rachitova–Stei area, where the primary (nonreworked) pyroclastic fragments are preponderant. The distinction

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between different types of fragments and their nomenclature has been made according to Le Bas and Streckeisen (1991), Gillespie and Styles (1999) and Seclaman et al. (1999). There has been evidenced a wide variety of crystal and rock fragments in the volcaniclastic deposits (see Table 1). The most abundant are (i) the primary pyroclastic fragments (pyroclasts), which resulted as a direct action of explosive volcanic activity (formed from cooling magma during transport prior to primary deposition or detached by explosion from previous rocks) and which have not been reworked by sedimentary processes. The pyroclasts include rock fragments (lithics), glass fragments, crystal fragments or individual crystals. In addition, there is a minor quantity of (ii) reworked pyroclastic fragments (which are a direct result of volcanic activity, but which have been reworked by sedimentary processes), and (iii) epiclastic fragments.

3.1. Pyroclasts 3.1.1. Rock fragments The average percentage of rock fragments ranges from 50% to 80% of total rock volume. The rock fragments are usually angular (common for coarse grains and lapilli) and subrounded (common for blocks and bombs). The average sizes of the rock fragments are given in Table 1. The following is a brief petrographic description of the lithic types, subdivided into volcanic and non-volcanic.

3.1.1.1. Volcanic lithics. There have been distinguished various volcanic pyroclastic fragments. According to their modal mineral content and based on the QAPF diagram (Le Bas and Streckeisen, 1991), there have been separated andesites, latite-andesites, latites, dacites and rhyolites. Usually, they are larger than 1 cm (up to 1 m) and commonly present a porphyritic fabric. The groundmass frequently has a fluidal texture and varies from vitreous to microcrystalline. The andesitic fragments are the most frequent and occur as coarse angular grains, angular lapilli or subrounded volcanic bombs. Their mineralogical composition consists mainly of plagioclase, which occurs either as phenocrysts or as laths in the groundmass. The phenocrysts of mafic minerals (clinopyroxene and amphibole) are also common, but less quantitatively than plagioclase. The phenocrysts of green hornblende are frequently replaced by brown hornblende or opaque minerals. Almost all the analysed andesites contain opaque minerals (i.e. magnetite, ilmenite) and apatite as accessory minerals. Some andesitic fragments contain xenoliths of pyroxenites or porphyritic microdiorites. In other andesitic fragments, particularly in those that are rich in clinopyroxene, gabbroic nodules have been noticed. In these nodules, calcite pseudomorphosis after a mafic mineral (probably olivine) with an external celadonitic rim can sometimes be observed. Taking into account the nature of the mafic mineral (cf. Gillespie and Styles, 1999), the andesites are classified as: amphibole andesites, biotite-bearing-amphibole andesites, pyroxenebearing-amphibole andesites, and pyroxene-andesites.

Table 1 The modal composition, the roundness and the size of the volcaniclastic fragments. (Categories of roundness used to describe fragments: VA = very angular; A = angular; SA = subangular; SR = subrounded, R = rounded; VR = very rounded). Petrographic types

COARSE TUFF

TUFF-BRECCIA

TUFFITE

Volcaniclastic fragment types

Coarse grains

Proportion (%), size (d), roundness (r) of the fragments:

%

d (mm)

r

%

d (mm)

r

%

d (mm)

r

%

d (mm)

r

%

d (mm)

r

10–50 15–20 30–60 10–50 5–15 0–20 – 0–5 0–2 0–10 1–10 0–1 0–1 0–1

0.5–2.0 0.5–1.0 0.5–2.0 0.5–2.0 0.5–1.0 0.5–1.0 – 0.4–0.8 0.2–0.5 0.6–1.5 0.4–1.2 0.5–0.6 0.8–1.0 0.5–2.0

A A A A A A – A A A A A A A

10–45 12–16 30–40 15–50 – – – 0–4 – 5–8 1–5 0–1 – –

0.5–2.0 0.5–1.0 0.5–1.5 0.5–2.0 – – – 0.4–0.5 – 0.7–1.0 0.4–1.2 0.2–0.4 – –

A A A A – – – A – A A A – –

10–15 10–12 5–10 4–8 – – 2–5 – – – – 0–2 – –

2–20 8–15 9–20 2–20 – – 7–9 – – – – 6–7 – –

A A A A – – A – – – – A – –

10–25 10–15 3–10 0–5 – – – – – – – – – –

75–230 80–200 75–200 60–90 – – – – – – – – – –

SR SR SR SR – – – – – – – – – –

10–45 10–20 20–45 15–45 – – – 0–2 – 2–8 – – – –

0.1–2.0 0.1–1.0 0.2–1.5 0.5–1.5 – – – 0.5–1.0 – 0.6–1.0 – – – –

A A A A – – – – – A – – – –

2–35 0–5 0–5 0–5 3–5 0–10 1–8 0–1 0.5–5 0.8–2 0–1

0.2–0.8 0.1–1.5 0.2–0.7 0.2–0.7 0.2–0.8 0.2–0.8 0.1–0.3 0.3–0.7 0.1–2.0 0.6–1.0 0.1–0.2

A A A A A A A A A A A

2–20 0–4 3–5 2–5 3–5 2–5 0–5 0–1 0.5–5 0.8–2 0–1

0.2–0.6 0.7–1.5 0.2–0.6 0.3–0.7 0.2–0.8 0.2–0.8 0.1–0.3 0.3–0.7 0.1–2.0 0.6–1.0 0.1–0.2

A A A A A A A A A A A

– – – – – – – – – – –

– – – – – – – – – – –

– – – – – – – – – – –

– – – – – – – – – – –

– – – – – – – – – – –

– – – – – – – – – – –

15–20 0–10 1–5 0–1 – 1–3 0–5 0–1 2–5 1–2 –

0.3–0.7 0.2–0.6 0.2–0.6 0.2–0.5 0.2–0.8 0.2–0.6 0.1–0.3 0.2–0.7 0.1–2.0 0.6–1.0 –

A A A A A A A A A A –

–

–

–

–

–

–

–

–

–

–

–

–

2–8

0.2–2.0

R

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – – –

0–4 10–15 – 0–5 15–20 0–1

0.2–1.0 0.5–2.0 – 0.2–0.5 0.5–1.8 0.5–1.2

A R – R R R

PRIMARY PYROCLASTIC FRAGMENTS Lithics Volcanic Amphibole andesites Biotite-bearing-amphibole andesites Pyroxene-bearing-amphibole andesites Pyroxene-andesites Latites and latite-andesites Rhyolites Dacites Plutonic Granites Gabbros Metamorphic Quartz-muscovite schists Microgranular quartzites Muscovite quartzites Graphitic phyllites Sedimentary Quartz-rich sandstones Crystals Volcanic Plagioclase Sanidine Brown hornblende Green hornblende Opaque hornblende Clinopyroxene Quartz Non-volcanic Biotite Muscovite Quartz Garnet REWORKED PYROCLASTIC FRAGMENTS Lithics Andesites, dacites EPLICLASTIC FRAGMENTS Crystals Biotite Quartz Lithics Plutonic Granites Metamorphic Quartzites, muscovite schists, marbles Sedimentary Sandstones

Coarse grains

Lapilli

Blocks/bombs

Coarse grains

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The latite-andesitic fragments usually consist of phenocrysts of plagioclase and hornblende in a hypocrystalline groundmass. The plagioclase is zoned and the hornblende is mostly opaque. The groundmass has a fluidal texture and contains sanidine and plagioclase laths. Some latite-andesites have a rich content in hornblende (bordered by opaque rims), with no trace of pyroxene or biotite; others do not contain mafic minerals. The latitic fragments have a porphyritic texture and a fluidal groundmass. Phenocrysts are represented by plagioclase, alkali-feldspar, biotite, ±opaque hornblende. The groundmass consists mainly of alkali-feldspar, plagioclase, and rarely clinopyroxene. The dacitic fragments are extremely rare and are found only as angular lapilli. They have a porphyritic fabric and consist generally of plagioclase, quartz, hornblende, pyroxene and biotite, in a felsic groundmass. The phenocrysts of quartz are corroded and seem to be assimilated. The rhyolitic fragments are common in the lower sequence of the volcaniclastic deposits, and occur as angular coarse grains and angular lapilli. Their composition consists mainly of plagioclase, sanidine, quartz, and biotite phenocrysts, in a zeolitised groundmass. The biotite is strongly deformed. 3.1.1.2. Non-volcanic lithics. Non-volcanic lithics are represented by fragments of plutonic (i.e. granites, gabbros), metamorphic and sedimentary rocks. The granites (<5% modal) have a hypidiomorphic, medium to coarse grained fabric, and consist of quartz, microcline, plagioclase (oligoclase) and biotite. The gabbros (<2% modal) have a coarse grained fabric and consist mainly of plagioclase (labradorite) and clinopyroxene. The most common metamorphic rock fragments observed are quartz-muscovite schists (<10% modal). Sometimes, the quartz-muscovite schists contain small quantities of albite and zoisite. Other metamorphic lithics include muscovite quartzites (<1% modal), microgranular quartzites (<10% modal) and graphitic phyllites (<1% modal). Sedimentary lithic fragments appear rarely (<1% modal), usually in the coarse tuffs, and petrographically, they are quartz-rich sandstones. 3.1.2. Individual crystals and crystal fragments 3.1.2.1. Volcanic crystal fragments. Volcanic crystal fragments barely exceed 1 mm and consist mainly of plagioclase, hornblende and clinopyroxene. More rarely, fragments of sanidine and quartz have been noticed (Table 1). The crystals and fragments of plagioclase are the most common. In the volcaniclastic rocks, the modal frequency of the plagioclase crystal fragments ranges from 2 to 35%. They are angular with low sphericity. The individual crystals of plagioclase have characteristic oscillatory zoning and vary in habit from euhedral to anhedral. The euhedral crystals have a compositional variation ranging from An4.9 in the rim to An60 in the core. The hornblende crystals (green or brown in thin sections) are euhedral to anhedral. Their composition is similar to the amphiboles from the andesitic pyroclastic fragments (i.e. edenite–hornblende to ferroan–pargasite–hornblende). Opaque hornblende crystals are also present, but much more rarely. Clinopyroxene occurs both as fragments of crystals or individual euhedral crystals, and has a chemical composition of augite. In some rocks its modal frequency comes to 10%. Sanidine occurs frequently as perfect euhedral crystals or twinned crystals whose 2 V angle ranges from 15° to 25°. Sanidine is common in the lower sequence of the volcaniclastic deposits and rare in the other sequences. The quartz fragments typically occur in the lower sequence of the volcaniclastic deposits, where they are frequently associated with sanidine. They do not exceed 8% of the rock volume. The individual crystals of quartz either have bipyramidal forms or are magmatic

309

corroded. Occasionally, there have been observed quartzitic polycrystalline aggregates, which are also magmatic corroded. 3.1.2.2. Non-volcanic crystal fragments. Non-volcanic crystal fragments are rare and include muscovite (<5%), quartz (<2%), biotite (<1%) and garnet (<1%). The muscovite and biotite crystals are frequently deformed, having kink bands. The quartz is also deformed, showing undulatory extinction. 3.2. Reworked pyroclastic fragments Reworked pyroclastic fragments occur more rarely compared to the pyroclasts. They occur especially in the upper member of the deposits and present similar mineralogical compositions to the volcanic pyroclastic fragments presented above. Characteristic for the reworked pyroclasts is their rounded form. 3.3. Epiclasts Epiclasts are commonly associated with reworked pyroclastic fragments and are significant only in some sequences of the volcaniclastic deposits. The most common epiclasts are given by fragments of quartz and by lithic fragments of quartzite and muscovite schist. In addition, there are small quantities of fragments of granite, marble and sandstone. In Table 1 are presented more details about percentage, size and roundness of the epiclasts. 4. Quantitative chemical analysis of minerals from the volcanic pyroclasts The quantitative chemical analyses of the minerals have been performed on five polished thin sections selected from the volcanic lithic pyroclasts using a Jeol Superprobe JXA-8600 electron microprobe controlled by a LINK–eXL system. Electron microprobe analyses of the minerals from the volcanic pyroclastic fragments are given in Tables 2–5 and Figs. 2 and 3. From a large set of data obtained, only a few chemical compositions of the minerals are presented. In andesites, the zoned phenocrysts of plagioclase (Figs. 2a and 3c) frequently present a homogenous core inside (An60), followed by a normaloscillatory zoned mantle and a thin rim, ranging in composition from An9.5 to An4.9. Chemically, this indicates an increasing of Si content and a decreasing of Al and Ca contents from core to rim. The small laths of plagioclase from the groundmass (Fig. 2c) have a similar composition with the rim of the zoned phenocrysts of plagioclase. The alkali-feldspar presents a wide range of sodium composition as can be observed in Table 4. The amphibole compositional range, in terms of a plot of Mg/Mg+ FeII vs. TSi, indicates an edenite–hornblende to ferroan– pargasite–hornblende composition (Fig. 3a). The opaque outer rim developed on hornblende phenocrysts (Fig. 2b) has a composition of magnetite. The magmatic biotite has predominantly a phlogopitic chemical composition (Fig. 3b). The clinopyroxene crystals (Fig. 2b,c,d) contain a small quantity of NaFeIIISi2O6 and Tschermak's molecules, MgSiO6, which is typical for the volcanic rocks. However, the major compounds are CaMgSi2O6 and CaFeSi2O6; the clinopyroxene having a composition of augite that lies within the range Wo38–48 En41–54 Fs7–19 (Fig. 3d). 5. Types of volcaniclastic rocks and their stratigraphic succession The volcaniclastic rocks from the Hateg Basin are miscellaneous assemblages of primary pyroclastic fragments with little reworked pyroclastic or epiclastic material. These fragments have variable sizes (see Table 1); the larger (>2 mm) being cemented by the coarse grained material (<2 mm). According to the IUGS scheme (Le Bas and Streckeisen, 1991) and the BGS recommendations (Gillespie and Styles, 1999), which use the

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Table 2 Chemical compositions of the selected samples of amphibole from the andesitic fragments analysed by electron microprobe. The formula has been calculated considering 23 atoms of oxygen. Sample

(%) SiO2 TiO2 Al2O3 Fet Cr2O3 MnO MgO CaO Na2O K2O Total TSi TAl Sum T Cal CCr CFe3+ CTi CMg CFe2+ Sum C BFe2+ BMn BCa Sum B ANa AK Sum A Cations O

D613

D613

D613

D613

D613

D613

D613

R332

R332

R332

R351

C1

C11

H27

H28

B6

H6

H7

P14

P15

A4

C18

44.74 1.16 10.92 15.34 0.03 0.45 12.44 10.50 1.60 0.56 97.74 6.61 1.39 8 0.51 0.00 0.05 0.13 2.74 1.56 5 0.28 0.06 1.66 2 0.46 0.11 0.56 15.56 23

42.96 2.10 12.46 13.61 0.02 0.31 12.64 11.09 1.84 0.74 97.77 6.34 1.66 8 0.51 0.00 0.01 0.23 2.78 1.46 5 0.21 0.04 1.75 2 0.53 0.14 0.67 15.67 23

44.08 1.29 11.44 15.83 0.00 0.49 12.10 10.50 1.68 0.53 97.94 6.52 1.48 8 0.51 0.00 0.11 0.14 2.67 1.58 5 0.28 0.06 1.66 2 0.48 0.10 0.58 15.58 23

46.52 1.12 9.37 14.77 0.01 0.53 13.31 10.55 1.38 0.42 97.98 6.82 1.18 8 0.44 0.00 0.03 0.12 2.91 1.50 5 0.28 0.07 1.66 2 0.39 0.08 0.47 15.47 23

44.61 1.00 10.72 15.70 0.01 0.47 12.18 10.71 1.58 0.63 97.61 6.62 1.39 8 0.49 0.00 0.10 0.11 2.69 1.61 5 0.24 0.06 1.70 2 0.45 0.12 0.57 15.57 23

47.23 0.95 8.86 14.55 0.02 0.49 13.67 10.44 1.32 0.35 97.88 6.91 1.09 8 0.44 0.00 0.01 0.11 2.98 1.47 5 0.30 0.06 1.64 2 0.37 0.07 0.44 15.44 23

46.08 1.10 10.16 15.06 0.01 0.40 13.19 10.47 1.61 0.49 98.57 6.73 1.27 8 0.48 0.00 0.00 0.12 2.87 1.53 5 0.31 0.05 1.64 2 0.46 0.09 0.55 15.55 23

41.45 2.94 13.02 12.97 0.00 0.20 12.80 11.04 2.13 1.37 97.92 6.16 1.84 8 0.44 0.00 0.00 0.33 2.84 1.40 5 0.22 0.03 1.76 2 0.61 0.26 0.87 15.87 23

41.70 2.84 12.78 13.51 0.00 0.17 12.67 10.86 2.05 1.37 97.95 6.20 1.80 8 0.44 0.00 0.00 0.32 2.81 1.43 5 0.25 0.02 1.73 2 0.59 0.26 0.85 15.85 23

41.91 2.97 11.07 14.76 0.00 0.30 12.64 10.90 1.97 1.42 97.94 6.27 1.73 8 0.22 0.00 0.01 0.33 2.82 1.63 5 0.22 0.04 1.75 2 0.57 0.27 0.84 15.84 23

42.75 2.31 11.41 13.73 0.02 0.22 13.44 10.60 2.91 0.51 97.90 6.35 1.65 8 0.34 0.00 0.00 0.26 2.98 1.42 5 0.29 0.03 1.69 2 0.84 0.10 0.94 15.94 23

amount of pyroclastic fragments for classification purposes, the volcaniclastic rocks from the Hateg Basin can be grouped in three categories: (1) pyroclastic rocks, which consist of more than 75% pyro-

clastic fragments; (2) tuffites, which contain 25% to 75% pyroclastic fragments; and (3) volcaniclastic sedimentary rocks, that contain more than 10% volcanic debris, but less than 25% pyroclastic fragments.

Table 3 Chemical compositions of the selected samples of clinopyroxene from the fragments of andesites analysed by electron microprobe. Sample

(%) SiO2 TiO2 Al2O3 FeO Cr2O3 MnO MgO CaO Na2O K2O Total TSi TAl TFe3 M1 Al M1 Ti M1 Fe3 M1 Fe2 M1 Cr M1 Mg M1 Ni M2 Mg M2 Fe2 M2 Mn M2 Ca M2 Na M2 K Cations

R332

R332

R332

R332

R332

R332

R333

R333

R333

R416

R416

B1

H14

P22

C6

E6

C7

A12

C4

C5

D1

E2

51.80 0.53 2.13 8.31 0.00 0.39 15.01 21.07 0.30 0.03 99.57 1.93 0.07 0 0.02 0.02 0.05 0.09 0.00 0.83 0 0.00 0.13 0.01 0.84 0.02 0 4

51.59 0.45 3.16 10.15 0.15 0.37 13.67 19.77 0.58 0.02 99.91 1.92 0.08 0 0.06 0.01 0.03 0.13 0.00 0.76 0 0.00 0.16 0.01 0.79 0.04 0 4

54.37 0.23 1.73 3.90 0.29 0.14 17.60 21.26 0.26 0.00 99.78 1.98 0.02 0 0.06 0.01 0.00 0.00 0.01 0.93 0 0.03 0.12 0.00 0.83 0.02 0 4

52.02 0.29 2.30 10.47 0.00 0.37 13.54 20.12 0.59 0.01 99.71 1.95 0.05 0 0.05 0.01 0.03 0.16 0.00 0.76 0 0.00 0.14 0.01 0.81 0.04 0 4

52.56 0.34 1.57 8.57 0.03 0.39 15.21 20.67 0.44 0.00 99.78 1.95 0.05 0 0.02 0.01 0.05 0.09 0.00 0.84 0 0.00 0.14 0.01 0.82 0.03 0 4

51.96 0.29 2.39 10.49 0.00 0.41 13.45 20.31 0.61 0.01 99.92 1.94 0.06 0 0.05 0.01 0.04 0.16 0.00 0.75 0 0.00 0.13 0.01 0.81 0.04 0 4

52.00 0.61 1.82 11.44 0.00 0.48 14.77 18.20 0.33 0.04 99.69 1.95 0.05 0 0.03 0.02 0.02 0.12 0.00 0.83 0 0.00 0.23 0.02 0.73 0.02 0 4

51.34 0.39 2.89 10.03 0.04 0.32 13.89 20.23 0.50 0.01 99.64 1.92 0.08 0 0.05 0.01 0.05 0.12 0.00 0.77 0 0.00 0.14 0.01 0.81 0.04 0 4

51.24 0.54 3.65 9.50 0.02 0.29 14.80 19.32 0.44 0.01 99.81 1.90 0.10 0 0.06 0.02 0.04 0.07 0.00 0.82 0 0.00 0.19 0.01 0.77 0.03 0 4

51.17 0.84 4.50 8.48 0.00 0.22 13.67 20.32 0.40 0.01 99.61 1.91 0.09 0 0.11 0.02 0.00 0.11 0.00 0.76 0 0.00 0.15 0.01 0.81 0.03 0 4

51.25 0.68 4.25 8.07 0.01 0.18 13.91 20.31 0.42 0.00 99.08 1.92 0.08 0 0.10 0.02 0.00 0.10 0.00 0.78 0 0.00 0.15 0.01 0.81 0.03 0 4

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Table 4 Chemical compositions of the selected samples of plagioclase (‘pl’) and alkali-feldspar (sanidine, ‘san’) from the volcanic lithic pyroclasts analysed by electron microprobe. Sample

(%) SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total Si Al Ti Fe2+ Mn Mg Ca Na K Cations X Z Ab An Or

D613

R416

R416

R416

R416

R351

R333

R351

D613

R351

R351

D11

D2

A3

E5

F7

F2

F4

A11

F7

A6

F1

san 62.75 0.02 17.81 0.59 0.03 1.05 1.54 2.18 10.68 96.65 11.83 3.96 0.00 0.09 0.01 0.30 0.31 0.80 2.57 19.86 15.79 4.07 21.70 8.50 69.90

san 63.25 0.65 17.15 3.20 0.05 0.40 1.49 5.49 7.01 98.69 11.73 3.75 0.09 0.50 0.01 0.11 0.30 1.98 1.66 20.12 15.57 4.55 50.30 7.50 42.20

san 63.61 0.66 17.22 3.22 0.01 0.46 1.58 4.42 8.16 99.34 11.75 3.75 0.09 0.50 0.00 0.13 0.31 1.58 1.92 20.03 15.59 4.45 41.50 8.20 50.40

san 63.33 0.50 17.42 2.30 0.04 0.35 1.35 4.71 8.12 98.12 11.79 3.82 0.07 0.36 0.01 0.10 0.27 1.70 1.93 20.04 15.68 4.36 43.60 6.90 49.50

san 64.11 0.72 17.89 1.96 0.04 1.14 1.77 3.78 8.39 99.80 11.71 3.85 0.10 0.30 0.01 0.31 0.35 1.34 1.96 19.91 15.66 4.26 36.80 9.50 53.70

pl 54.80 0.01 28.38 0.32 0.00 0.03 11.63 4.59 0.20 99.96 9.90 6.04 0.00 0.05 0.00 0.01 2.25 1.61 0.05 19.90 15.94 3.96 41.20 57.60 1.20

pl 53.22 0.02 28.77 0.73 0.01 0.11 12.36 4.38 0.28 99.88 9.69 6.17 0.00 0.11 0.00 0.03 2.41 1.55 0.07 20.02 15.86 4.17 38.40 59.90 1.60

pl 53.84 0.03 28.94 0.22 0.03 0.03 12.07 4.57 0.14 99.87 9.76 6.18 0.00 0.03 0.01 0.01 2.34 1.61 0.03 19.96 15.94 4.03 40.30 58.90 0.80

pl 67.79 0.13 19.44 0.17 0.02 0.13 0.92 10.81 0.45 99.86 11.90 4.02 0.02 0.03 0.00 0.03 0.17 3.68 0.10 19.96 15.94 4.02 93.10 4.40 2.60

pl 54.71 0.02 28.28 0.35 0.01 0.02 11.39 4.88 0.18 99.84 9.90 6.03 0.00 0.05 0.00 0.01 2.21 1.71 0.04 19.95 15.93 4.02 43.20 55.70 1.10

pl 56.39 0.00 27.05 0.24 0.01 0.03 10.21 5.61 0.27 99.81 10.17 5.75 0.00 0.04 0.00 0.01 1.97 1.96 0.06 19.96 15.92 4.04 49.10 49.40 1.60

Considering the nature of the dominant pyroclastic fragments, the coarse tuffs have been separated in: (a) amphibole andesitic coarse tuffs, those which contain mainly pyroclasts of amphibole andesites, biotite-bearing-amphibole andesites and/or pyroxene-bearing-amphibole andesites; (b) pyroxene andesitic coarse tuffs, containing mainly pyroclasts of pyroxene-andesites; and (c) rhyolitic coarse tuffs, containing mainly rhyolitic pyroclasts. After Schmid (1981), the amphibole andesitic coarse tuffs and pyroxene andesitic coarse tuffs can be classified as lithic tuffs (consisting mainly of rock fragments)

and the rhyolitic coarse tuffs as vitric tuffs (consisting mainly of glass fragments). Using the same criteria the tuff-breccia has been classified in: (a) amphibole andesitic tuff-breccia (containing mainly pyroclasts of amphibole andesites, biotite-bearing-amphibole andesites and/or pyroxene-bearing-amphibole andesites) and (b) pyroxene andesitic tuff-breccia (containing mainly pyroclasts of pyroxene-andesite). The matrix of the coarse tuffs and tuff-breccia usually consists of coarse grained pyroclastic material (frequent plagioclase and glass) that is frequently altered. It often has vesicular spaces that are filled

Table 5 Chemical compositions of the selected samples of the magmatic biotite from the andesitic fragments analysed by electron microprobe. The number of cations has been calculated considering 24 atoms of oxygen per formula. Sample

(%) SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total Si AlIV AlVI Ti Fe2+ Cr Mn Mg Ca Na K Cations O Fe/(Fe + Mg) Mg/(Fe + Mg)

D613

D613

D613

D613

D613

D613

D613

D613

D613

D613

D613

H1

B10

H10

C12

C13

F13

F14

C3

B8

B9

H9

35.89 3.25 14.89 0.02 14.46 0.18 14.51 0.24 0.48 6.45 90.37 5.82 2.18 0.67 0.40 1.96 0.00 0.03 3.51 0.04 0.15 1.34 16.10 24 0.36 0.64

35.47 3.28 15.08 0.01 14.14 0.21 15.61 0.35 0.53 5.79 90.47 5.73 2.27 0.60 0.40 1.91 0.00 0.03 3.76 0.06 0.17 1.19 16.11 24 0.34 0.66

36.53 3.16 15.29 0.03 14.19 0.20 16.54 0.27 0.51 5.47 92.19 5.76 2.24 0.60 0.38 1.87 0.00 0.03 3.89 0.05 0.16 1.10 16.07 24 0.32 0.68

37.60 3.36 15.00 0.03 16.65 0.18 13.50 0.13 0.45 7.24 94.14 5.92 2.09 0.69 0.40 2.19 0.00 0.02 3.17 0.02 0.14 1.45 16.09 24 0.41 0.59

37.78 3.31 14.87 0.02 16.38 0.19 13.84 0.15 0.55 7.18 94.27 5.93 2.07 0.67 0.39 2.15 0.00 0.03 3.24 0.03 0.17 1.44 16.11 24 0.4 0.6

36.53 3.22 15.00 0.00 14.47 0.24 16.32 0.50 0.48 4.67 91.43 5.79 2.21 0.59 0.38 1.92 0.00 0.03 3.86 0.09 0.15 0.95 15.97 24 0.33 0.67

35.99 2.93 14.32 0.02 16.35 0.16 14.71 0.21 0.43 6.03 91.15 5.83 2.17 0.56 0.36 2.22 0.00 0.02 3.55 0.04 0.14 1.25 16.13 24 0.38 0.62

35.13 3.37 14.82 0.04 15.24 0.19 14.57 0.23 0.50 6.58 90.67 5.73 2.28 0.57 0.41 2.08 0.01 0.03 3.54 0.04 0.16 1.37 16.20 24 0.37 0.63

35.61 3.97 14.74 0.01 15.27 0.21 14.00 0.65 0.54 6.43 91.43 5.75 2.25 0.56 0.48 2.06 0.00 0.03 3.37 0.11 0.17 1.33 16.11 24 0.38 0.62

35.19 3.30 15.43 0.00 14.42 0.31 16.70 0.30 0.57 5.17 91.39 5.62 2.38 0.52 0.40 1.93 0.00 0.04 3.98 0.05 0.18 1.05 16.14 24 0.33 0.67

36.17 5.92 13.85 0.02 13.58 0.19 14.78 2.98 0.44 4.73 92.66 5.70 2.30 0.27 0.70 1.79 0.00 0.03 3.47 0.50 0.13 0.95 15.85 24 0.34 0.66

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Fig. 2. Electron microprobe photos of thin sections displaying the common minerals from the andesites. (a) Phenocryst of zoned plagioclase presenting a core inside followed by a normal-oscillatory zoned mantle and a thin rim. (b) Hornblende phenocryst bordered by an outer rim of magnetite. (c,d) Small laths of plagioclase and clinopyroxene crystals. [pl = plagioclase; hbl = hornblende; mt = magnetite; cpx = clinopyroxene; cel = celadonite presented in detail in the diagenetic minerals section].

with secondary minerals. Locally, the matrix (>60% modal) predominately consists of well cemented vitroclasts with a fluidal texture, forming a hyaline compact mass that encloses anhedral crystals of zoned plagioclase, clinopyroxene, acicular apatite, muscovite, fragments of gabbros and fragments of quartz-muscovite schists. The pyroclastic rocks are widespread in the Rachitova–Stei unit and constitute either well-sorted (coarse-tuff) deposits or poorly-sorted (tuff-breccia) deposits. The amphibole andesitic coarse tuffs, the pyroxene andesitic coarse tuffs, the amphibole andesitic coarse tuff-breccia and pyroxene andesitic tuff-breccia crop in this unit, in alternating sequences (Fig. 4a,b) with the tuffites (i.e. andesite-bearing tuffaceous conglomerates and andesite-bearing tuffaceous sandstones) and with the volcaniclastic sedimentary rocks (i.e. volcaniclastic conglomerates and volcaniclastic sandstones). The rhyolitic coarse tuffs occur in the lower sequence of the volcaniclastic deposits and can be better observed in the Rachitova tower area. As it is shown in the lithologic columns from Fig. 4, the volcaniclastic rocks from the Rachitova–Stei unit have predominantly an andesitic nature. The rhyolitic rocks are quantitatively subordinated, representing less than 5% from the total volume of the lithologic column. 6. Geochemistry of the volcanic pyroclastic fragments Major oxides and trace elements of fourteen unaltered samples of the primary volcanic pyroclastic fragments of andesites, latite-andesites and latites have been determined using X-ray fluorescence (XRF) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results are summarized in Table 6. Selected elemental data was plotted as graphs (Figs. 5 and 7) in order to reveal the petrochemical

and tectonic trends of the samples based on their bulk elemental chemistry. According to the total alkali content vs. silica content diagram (Fig. 5a), the volcanic pyroclastic fragments from the Hateg Basin, which have been diagnosed above as andesites, latite-andesites and latites on modal composition basis, correspond to the andesitic and trachyandesitic petrochemical types. 7. Diagenetic minerals The diagenetic minerals that are commonly observed in the volcaniclastic rocks from the Hateg Basin are celadonite, zeolites and calcite. In addition, there has been noticed rare secondary quartz. The celadonite is the most common and appears in all the volcaniclastic sequences giving a green colour to the volcaniclastic rocks from the Hateg Basin. The celadonite occurs either in the lithic fragments or in the matrix from the coarse tuffs, tuff-breccia, and tuffites (see Fig. 2b,c,d). It cements the coarse pyroclastic material, replaces the hornblende, clinopyroxene and volcanic glass fragments, or more frequently, it fills the vesicular cavities, presenting a fibro-radial or concentric fabric. In vesicles, the celadonite is sometimes associated with calcite and rarely with quartz or zeolites. In the bi-mineral vesicular assemblages, the celadonite appears either in the rim (e.g. bordering a calcite core) or forming concentric recurrences. In vesicles, the celadonite colour is usually dark-green to light-green. However, it becomes brown in highly oxidized rocks. The quantitative chemical analyses of the celadonite were performed by electron microprobe. The results are given in Table 7 and Fig. 6. Its formula, normalized to 12 tetrahedral and octahedral cations and 22 atoms of oxygen, indicates a composition between Mg-celadonite (K2Mg2Fe3+2Si8O20(OH)4) and ferro-aluminoceladonite

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Fig. 3. Composition of minerals from the volcanic lithic fragments represented in classification diagrams: (a) amphibole (after Leake et al., 1997), (b) biotite (after Deer et al., 2001), (c) feldspar (after Deer et al., 2001) and (d) pyroxene (after Morimoto et al., 1989).

(K2Fe2+2Al2Si8O20(OH)4). The Mg/(Mg + Fe2+) ratio varies from 0.34 to 0.61, while Al/(Al + Fe3+) ranges from 0.19 to 0.94. These chemical variations are correlated with the colour variations of celadonite, with dark-green related to a low content of Mg and Al and light-green related to a higher content of Mg and Al. The calcite occurs in irregular patches that replace the matrix of the pyroclastic rocks or, more frequently, as pseudomorphosis after

phenocrysts of plagioclase and mafic minerals (pyroxene, olivine?, hornblende). The calcite also fills vesicles in a concentric texture. The most common zeolites identified by X-ray diffraction and microscopic investigations are clinoptilolite and stilbite. The clinoptilolite is mainly formed on the vitreous mass and on the plagioclase fragments, and more rarely in the vesicles bordered by celadonite. In some andesitic fragments, the vitreous groundmass is completely

Fig. 4. Lithologic columns of the volcaniclastic rocks from the Rachitova–Stei unit: (a) in the Stei village area (b) near the Rachitova tower. The basement of the lithologic sequences can be observed only in (b), whereas the upper sequences are well expressed in (a).

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Table 6 Bulk chemical composition of the primary volcanic pyroclastic fragments from the Hateg Basin obtained by XRF (for major elements) and ICP-AES (for trace elements). FeO and Fe2O3 have been recalculated based on Irvine and Baragar (1971). X and Y were calculated using the formulas given in Fig. 7. The liquidus temperatures (Tliq.) of magma were inferred according to Sisson and Grove (1993). Sample

D57

R1b

R4a

D64

D67

D66

R2b

D611

wt.%

Andesites

SiO2 TiO2 Al2O3 Fe2O3 FeO Fet MnO MgO CaO Na2O K2O P2O5 LOI Total

64.71 0.57 14.78 2.07 3.25 5.32 0.04 3.06 4.81 2.13 1.34 0.15 5.50 102.11

57.10 0.92 17.19 2.42 5.41 7.83 0.08 4.60 5.49 2.91 1.69 0.24 4.72 102.77

58.41 0.70 17.09 2.20 4.81 7.01 0.11 3.20 3.79 3.83 1.71 0.20 3.36 99.62

56.35 0.79 14.26 2.29 4.85 7.14 0.12 3.44 7.56 1.62 2.48 0.18 6.59 100.28

55.90 0.74 16.85 2.24 4.45 6.69 0.09 4.01 5.06 4.27 1.75 0.24 5.02 100.17

58.47 0.56 17.54 2.06 3.47 5.53 0.09 3.20 5.75 2.89 2.67 0.17 3.21 99.72

57.71 0.80 15.78 2.30 4.56 6.86 0.10 3.71 5.06 3.94 1.65 0.25 4.13 99.55

55.30 0.69 15.50 2.19 4.37 6.56 0.09 4.38 5.84 3.24 1.77 0.24 5.61 98.76

50.89 0.82 17.55 2.32 5.24 7.56 0.09 3.95 6.68 3.26 2.13 0.52 7.08 100.28

61.25 0.55 17.21 2.05 3.68 5.73 0.05 2.93 4.80 3.17 2.09 0.16 4.49 102.01

ppm Sc V Cr Ni Cu Zn Ga Rb Sr Y Zr Nb Ba La Ce Nd Eu Yb Pb Th U

10.24 96.54 23.5 12.8 12.9 42.4 15.2 34.6 463 15.3 126 7.28 399 18.66 27.46 15.94 0.814 1.39 16.1 6.3 2.3

14.95 143.9 44.8 15.6 10.2 81.2 18.3 35.4 663 22.5 139.9 11.73 489 23.43 44.16 18.04 1.018 1.797 18.3 7.61 0

14.04 102.8 50.4 17.9 11.8 69.1 17.4 55.3 416 21.1 135.6 9.09 319 20.16 39.4 14.53 0.792 1.906 17.7 7.22 1.9

16.8 142.9 107.5 25.6 14 66.3 16.2 103.8 411 23.1 137.6 8.84 371 23.75 48.35 17.71 1.082 2.142 19.4 8.78 0

14.96 138 58.2 18.8 10.4 60.5 16.9 41.2 792 20.9 153.5 9.84 507 25.23 47.52 21.37 1.225 2.045 20.3 9.71 0

13.15 100.2 35.5 11.7 7.6 61.5 17.1 110.2 504 17.4 124.5 9.52 337 20.37 39.33 13.17 0.865 1.849 17.1 6.66 0

17.46 142 45.1 20.7 13.6 80.2 17.5 39.2 335 23.3 140.6 7.7 415 26.21 51.62 18.92 0.998 2.235 23.6 9.61 2.2

16.87 112.1 63.9 18.2 11.2 60.2 15.8 35.4 665 18.6 150.1 9.69 494 28.06 50.69 23.17 1.237 1.883 22.9 10.3 3.1

15.76 144.6 20.1 14.3 22 70.3 17.4 53.6 810 32.4 155.4 7.35 447 31.27 44.11 28.16 1.727 3.044 16.5 7.52 6.3

X Y T Liq(°C)

0.95 − 0.03 1008

2.81 − 0.32 1071

2.64 − 0.25 974

1.56 − 2.17 922

3.60 1.18 980

1.98 − 2.25 1029

3.27 1.00 975

3.12 1.94 919

5.64 1.79 935

Latite-andesites

D624

D618

D51a

D613

D68

D621

54.60 0.65 17.18 2.15 4.42 6.57 0.10 3.80 6.83 3.12 2.46 0.19 4.32 99.20

55.87 0.71 18.17 2.21 3.88 6.09 0.08 3.48 6.46 3.77 4.13 0.24 2.53 101.08

56.11 0.75 19.24 2.25 5.05 7.30 0.13 1.93 4.35 5.15 3.36 0.28 2.21 100.36

51.12 0.85 18.07 2.35 5.36 7.71 0.11 3.63 7.12 2.62 2.54 0.23 6.80 100.32

11.57 85.4 27.9 16.7 19.6 43.9 17.1 65.6 541 16 144.1 8.1 444 30.03 53.29 21.04 0.962 1.639 24.6 9.87 0

15.19 132.9 49.8 23.8 22.4 57.1 17.3 80.2 596 19.2 143.3 9.19 514 24.58 34.11 19.47 0.983 1.953 20.2 8.24 2.7

15.36 129.5 112.2 24.2 15 69 17.6 139.7 543 15.7 144.1 10.63 619 22.98 32.48 18.03 1.12 1.604 31.9 7.91 4.4

11.1 148.5 5.6 6.2 4.5 82.6 17 112.3 559 21 186.6 11.56 705 33.85 68.44 27.93 1.086 2.257 19 12.7 3.2

16.78 158.4 13.7 10.3 40.2 64.3 17.2 52.5 730 24.9 152.5 7.99 494 27.39 57.46 26.62 1.712 2.528 16.4 8.91 0

1.95 − 2.13 1047

2.84 − 1.78 994

3.35 − 4.66 1101

4.11 − 5.00 1056

2.89 − 2.18 957

Latites

replaced by clinoptilolite. The stilbite is rare and usually occurs in veins. The diagenetic quartz occurs very seldom and crystallises in vesicles as polygonal grains. It is either associated with celadonite, or formed as pseudomorphosis after euhedral crystals of amphiboles. 8. Conclusions and discussions 8.1. Nature and origin of the magma The petrographic investigations of the volcaniclastic rocks evidence a high content of pyroclastic fragments that resulted as a direct action of volcanic activity and which have not been reworked by sedimentary processes. Moreover, the high percentage of pyroclasts and the rich mafic content indicate a highly explosive volcanism. The explosive manifestation of the volcanism and the presence of hydrated mafic minerals (amphibole, ±biotite) in all strata of the volcaniclastic rocks indicate a high-H2O content of the magma. This feature of the magma is also evidenced indirectly by the relatively low liquidus temperatures (below 1200 °C) of the andesitic magma (see Table 6) inferred from the whole-rock geochemical composition (cf. Sisson and Grove, 1993). In the lithologic columns presented above, it can be observed that each stratum is likely to correspond to a volcanic eruption episode.

There have been distinguished at least twelve different successive explosions with variable intensities. The magma involved in the eruptions was mainly andesitic. In addition, the rhyolitic sequence from the bottom of the lithologic succession indicates that acidic lava erupted in a short episode at the beginning of the volcanic manifestation. It is commonly accepted that andesitic magma is associated with convergent plate boundaries and the igneous activity triggered by the subduction of an oceanic plate (c.f. Philpotts, 1990). The subduction first generates the prograde metamorphism of the oceanic crust, which becomes very efficient at 75–250 km depth. The aqueous phases derived by low-dehydrations ascend through the crust and determine partial melting of the overlying mantle-wedge. Theoretically, the overlying mantle-wedge can be overlain either by an oceanic crust or by a relatively thin continental crust (c.f. Middlemost, 1985; Philpotts, 1990). The modal and chemical composition of the upper mantle beneath the continent is likely to differ from that of the mantle beneath the ocean and island-arcs (Saunders et al., 1980) and therefore, these differences should be reflected in the chemical composition of the andesitic magma generated by subduction. Additionally, many studies have shown that the major and trace element geochemistry of volcanic rocks reflects provenance differences that depend upon the tectonic setting of the volcanism (e.g., Bhatia and Crook, 1986; Roser and Korsch, 1986). Trace elements are

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Fig. 5. Diagrams showing (a) total alkali vs. silica content, (b) SiO2 vs. Zr/TiO2, (c) Zr/TiO2 vs. Nb/Y and (d) Zr vs. TiO2 (based on Cox et al., 1979; Winchester and Floyd, 1977). The samples from the volcanic pyroclastic fragments follow an andesitic trend.

particularly useful in this regard, especially those that are relatively immobile (e.g. La, Th, Sc etc.). These elements have low geochemical mobility during sedimentary processes of weathering, transportation,

and burial (McLennan et al., 1983), and also during diagenetic and very low grade metamorphic reconstitution (Roser et al., 2002). Hence, the ratio and abundance of immobile trace elements in the

Table 7 Chemical composition of the selected samples of celadonite analysed by electron microprobe. The number of cations has been calculated considering 22 atoms of oxygen per formula. Fe3+/Fe2+ ratio has been calculated from the charge balance. Sample

(%) SiO2 TiO2 Al2O3 Cr2O3 Fet MnO MgO CaO Na2O K2O Total Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K Cations Mg/(Mg + Fe2+) Al/(Al + Fe3+)

R332

R332

R332

R332

R332

R332

R332

R332

R332

R333

R333

D1

D2

E4

F6

G11

K5

P18

P19

P20

C2

C1

52.37 0.03 3.70 0.03 17.94 0.10 6.08 0.07 0.12 9.88 90.34 8.11 0.00 0.68 0.00 1.28 1.05 0.01 1.40 0.01 0.04 1.95 14.54 0.57 0.35

52.34 0.03 3.62 0.00 18.25 0.07 5.91 0.05 0.21 9.76 90.24 8.12 0.00 0.66 0.00 1.27 1.10 0.01 1.37 0.01 0.06 1.93 14.54 0.55 0.34

52.92 0.34 6.60 0.04 17.84 0.06 5.80 0.15 0.25 9.02 93.02 7.87 0.04 1.16 0.00 0.55 1.67 0.01 1.29 0.02 0.07 1.71 14.40 0.44 0.68

53.89 0.02 2.03 0.00 19.55 0.06 6.33 0.07 0.09 9.95 91.99 8.25 0.00 0.37 0.00 1.58 0.93 0.01 1.44 0.01 0.03 1.94 14.55 0.61 0.19

49.28 0.10 6.64 0.00 16.36 0.05 6.20 0.32 0.27 8.13 87.35 7.79 0.01 1.24 0.00 0.40 1.76 0.01 1.46 0.05 0.08 1.64 14.44 0.45 0.76

52.76 0.36 4.76 0.02 17.40 0.07 5.70 0.11 0.13 9.43 90.74 8.07 0.04 0.86 0.00 0.98 1.24 0.01 1.30 0.02 0.04 1.84 14.40 0.51 0.47

53.97 0.06 3.90 0.02 16.74 0.04 6.23 0.09 0.33 9.78 91.16 8.19 0.01 0.70 0.00 1.20 0.93 0.01 1.41 0.01 0.10 1.89 14.45 0.60 0.37

52.70 0.02 3.86 0.00 17.01 0.10 5.77 0.04 0.14 9.43 89.06 8.20 0.00 0.71 0.00 1.17 1.05 0.01 1.34 0.01 0.04 1.87 14.40 0.56 0.38

50.83 0.03 4.01 0.00 16.61 0.06 5.74 0.09 0.18 9.12 86.68 8.14 0.00 0.76 0.00 1.11 1.12 0.01 1.37 0.02 0.05 1.86 14.44 0.55 0.41

52.97 0.01 7.39 0.02 17.79 0.09 5.36 0.29 0.36 7.32 91.59 7.91 0.00 1.30 0.00 0.09 2.13 0.01 1.19 0.05 0.10 1.39 14.19 0.36 0.93

52.91 0.01 7.44 0.03 17.99 0.02 5.12 0.27 0.32 7.31 91.40 7.92 0.00 1.31 0.00 0.00 2.25 0.00 1.14 0.04 0.09 1.40 14.17 0.34 0.94

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Fig. 6. Octahedral cation contents for the electron microprobe analyses of celadonite are plotted predominantly in the Mg-celadonite (K2Mg2Fe3+2Si8O20(OH)4) field (diagram based on Li et al., 1997).

volcanic fragments are reliable signatures inherited from the igneous material. Whole-rock composition and immobile trace elements Th, La, Sc ratio from the pyroclastic fragments of the Hateg Basin are plotted in the island arc fields of the tectonic-setting discrimination diagrams

(Fig. 7). Based on the SiO2 vs. K2O/Na2O diagram (c.f. Roser and Korsch, 1986), the Hateg Basin volcanism corresponds to the oceanic island arc field (Fig. 7a), being plotted very close to the active continental margin field. The projections of analysed samples in the diagrams of Bhatia from Fig. 7c,d (Bhatia, 1983; Bhatia and Crook, 1986) also fall into the island arc fields (corresponding to the continental island arc and oceanic island arc fields, plotted near the active continental margin field). Likewise, the discrimination diagram that uses the immobile trace elements Th, La, and Sc to discern between tectonic settings (Bhatia and Crook, 1986; Condie, 1993) indicates a continental island arc tectonic environment (Fig. 7b). The presence of the rhyolitic episode in the volcaniclastic sequences from the Hateg Basin, which appears during the first stages of eruption, is also relevant for establishing the geotectonic setting of the magmatism. Most probably, rhyolites occur where the overriding plate is of continental material; they are almost totally lacking where only oceanic plates are involved. Partial melting or assimilation of crustal material would seem a likely explanation for this fact (Philpotts, 1990). A more convincing proof that the andesitic magma ascended through a continental crust is given by the presence of non-volcanic pyroclastic fragments derived from continental crustal rocks (i.e. granites, quartzmuscovite schists, muscovite quartzites, graphitic phyllites etc.). These fragments are detached from the continental crust as a result of an explosive volcanic action. Therefore, the petrographic and geochemical characteristics of the volcaniclastic rocks from the Hateg Basin can be well enlightened only if it is admitted that the magma has been generated by the subduction of an oceanic plate under a thin continental plate.

Fig. 7. Discrimination diagrams for inferring the tectonic setting based on whole-rock geochemical composition of the volcanic pyroclastic fragments from the Hateg Basin. (a) SiO2 vs. K2O/Na2O plot; (b) La-Th-Sc plot; (c) (Fe2O3 + MgO) − TiO2 plot; (d) X = 0.303 − 0.0447 × SiO2 − 0.972 × TiO2 + 0.008 × Al2O3 − 0.267 × Fe2O3 + 0.208 × FeO − 3.082 × MnO + 0.14 × MgO + 0.195 × CaO + 0.719 × Na2O − 0.032 × K2O + 7.51 × P2O5 vs. Y = 43.57 − 0.421 × SiO2 + 1.988 × TiO2 − 0.526 × Al2O3 − 0.551 × Fe2O3 − 1.61 × FeO + 2.72 × MnO + 0.881 × MgO − 0.907 × CaO − 0.177 × Na2O − 1.84 × K2O + 7.244 × P2O5; (figure a is based on Roser and Korsch, 1986; figure b is based on Bhatia and Crook, 1986; Condie, 1993; figures c. and d. are based on Bhatia, 1983, Bhatia and Crook, 1986) [PM: Passive Margin, ACM: Active Continental Margin, CIA: Continental Island Arc, OIA: Oceanic Island Arc].

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Acknowledgments

It is generally accepted that igneous activity does not occur at the convergent boundaries, but is located in the overriding plate some distance behind the boundary. Therefore, the volcanic eruptions that generated the volcaniclastic rocks from the Hateg Basin must have taken place on a continental plate, signifying that the explosion centre was geographically situated either on a continent or on a continental island arc. As shown above, the pyroclastic fragments are mainly angular, which indicates a very short distance of transportation and a deposition relatively close to the explosion centre. It is difficult to find objective criteria that can be used to measure the actual distance between the centre of explosion and the place where the pyroclastic fragments were deposited. However, considering the relatively high frequency in blocks that reach tens of kilograms, it can be admitted that the distance between the explosion centre and the depositional environment probably did not exceed 15–20 km. The nature of the depositional environment is unclear. According to Anastasiu (1991), Grigorescu (1992), Grigorescu et al. (1994) the pyroclastic rocks were deposited in an aquatic continental basin. The stratification of the volcaniclastic deposits and their poor sorting indicate a deposition in an aquatic basin. However, the most important diagenetic transformations of the volcanic material in zeolites and celadonite can be explained only by admitting that the pyroclastic fragments have interacted with sea water during diagenesis. The age of these diagenetic processes is difficult to be inferred, and it is possible that the diagenesis has taken place later. There are small relicts of Miocene limestone in the Rachitova area, and therefore the hypothesis that the volcaniclastic deposits from the Rachitova–Stei unit came in contact with sea water is not excluded.

The authors are grateful to all the editorial board, including Dr. Csiki Zoltan, for the invitation to participate at this special issue which brings together the most important scientific results regarding the Hateg Basin geology. This research was conducted as part of the first author's postdoc research at the University of Bucharest, Romania and University of Salzburg, Austria, and was financially supported by the National University Research Council, Grant 1677. The authors especially thank Professor Dan Grigorescu for his scientific support and valuable coordination in developing the field and laboratory investigations. The authors greatly appreciate the time and effort of all the participants in the fieldwork campaigns, and thank Mihaela Magopat for conducting the XRF and ICP-MS analyses at the National Superior School of Mines from Saint-Etienne and especially thank Dr. Dan Topa for his supportive professional assistance during the electron microprobe analyses at the University of Salzburg. We are also grateful to the reviewers for their constructive and thoughtful observations which have improved the manuscript.

8.3. Location of the explosion centre The present volume of volcanic material which exists in the Rachitova–Stei unit is at least 1 km3, but the total quantity of the extruded material must have been much more substantial. Therefore, it seems reasonable to suppose that the volcanic explosion had a centre of considerable dimensions, where either andesites or their subvolcanic equivalent (dioritic rocks) should be found. However, there is no evidence of such a volcano neither in the Hateg Basin nor anywhere else in the South Carpathians. Hypothetically, the volcanic centre would be more easily determined if the subduction slab that generated the andesitic magmatism could be located . But there is no evidence of an oceanic plate under thin continental plate subduction in the South Carpathians in Late Cretaceous. It is commonly accepted that the South Carpathians had a complex structure already established in Late Cretaceous, with at least two units tectonically superposed, respectively the Getic Unit and the Danubian Unit (Iancu et al., 2005). The two units, consisting mainly of metamorphic and magmatic rocks, make up a thick continental crust, and not a thin continental crust. A hypothesis that might answer the explosion centre issue is that the present tectonic units of the Hateg Basin, that bear volcaniclastic rocks, are fragments displaced from another geotectonic context, different from the South Carpathians geological setting. This was most likely a continental island arc basin located near an active continental margin. This hypothesis is supported by the following arguments: (1) the volcaniclastic units of the Hateg Basin are tectonically delimitated from the South Carpathians by strike-slip faults and (2) the paleogeographic reconstructions, based on paleomagnetic analyses (Patrascu and Panaiotu, 1990; Patrascu et al., 1993; Panaiotu and Panaiotu, 2002), place the volcanic rocks from the Hateg Basin thousands of kilometres away, somewhere at 20–30° North latitude, in the Mediterranean Tethys Ocean during the Maastrichtian.

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