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Lithos 104 (2008

Combining trace-element compositions, U–Pb geochronology and Hfisotopes in zircons to unravel complex calcalkaline magma chambers

in the Upper Cretaceous Srednogorie zone (Bulgaria)

I. Peytcheva a,b,⁎, A. von Quadt a, N. Georgiev c, Zh. Ivanov c, C.A. Heinrich a, M. Frank a,1

a Institute of Isotope Geochemistry and Mineral Resources, ETH-Zurich; 8092-Zurich, Switzerlandb Central laboratory for mineralogy and crystallography, BAS, 1113-Sofia, Bulgaria

c Faculty of Geology and Geography, Sofia University, 1000 Sofia, Bulgaria

Received 18 January 2007; accepted 31 January 2008Available online 15 February 2008

Abstract

Precise U–Pb geochronology and Hf isotope tracing of zircon is combined with whole-rock geochemical and Sr and Nd isotope data in order tounravel processes affecting mafic to felsic calcalkaline magmas prior to and during their crystallization in crustal magma chambers along thesouthern border of Central Srednogorie tectonic zone in Bulgaria (SE Europe). ID-TIMS U–Pb dating of single zircons from felsic and mixed/mingled dioritic to gabbroic horizons of single plutons define crystallization ages of around 86.5–86.0, 85.0–84.5 and 82 Ma. Concordia ageuncertainties are generally less than 0.3 Ma (0.35%–2σ), and as good as 0.08 Ma (0.1%), when the weighted mean 206Pb/238U value is used. Suchprecision allows the distinction of magma replenishment processes if separated by more than 0.6–1.0 Ma and when they are marked by newlysaturated zircons. We interpret zircon dates from a single sample that do not overlap to reflect new zircon growth during magma recharge in along-lived crustal chamber. Mingling/mixing of the basaltic magma with colder granitoid mush at mid- to upper-crustal levels is proposed toexplain zircon saturation and fast crystallization of U- and REE-rich zircons in the hybrid gabbro.

Major and trace-element distribution and Sr and Nd whole-rock isotope chemistry define island arc affinities for the studied plutons. Slabderived fluids and a sediment component are constrained as enrichment sources for the mantle wedge-derived magma, though Hf isotopes inzircon suggest crustal assimilation was also important. Inherited zircons, and their corresponding ε-Hf, from the hybrid gabbroic rocks trace thelower crust as possible source for enrichment of the mantle magma. These inherited zircons are about 440 Ma old with ε-Hf of −7 at 82 Ma,whereas newly saturated concordant Upper Cretaceous zircons reveal mantle ε-Hf values of +7.2 to +10.1. The upper and middle crustscontribute in the generation of the granitoid rocks. Their zircon inheritance is Lower Palaeozoic or significantly older and crustal dominated with82–85 Ma corrected ε-Hf values of −28. The Cretaceous concordant zircons in the granitoids are mantle dominated with a ε-Hf values spreadingfrom +3.9 to +7.© 2008 Elsevier B.V. All rights reserved.

Keywords: Magma mixing/mingling; Upper Cretaceous; Srednogorie; U–Pb zircon dating; ε-Hf; Nd–Sr isotopes

⁎ Corresponding author. Institute of Isotope Geochemistry and MineralResources, ETH-Zurich; 8092-Zurich, Switzerland. Tel.: +41 44 632 3724; fax:+41 44 632-1827.

E-mail addresses: [email protected] (I. Peytcheva),[email protected] (A. von Quadt).1 Now at: IFM-GEOMAR, Leibniz Institute for Marine Sciences at the

University of Kiel, Germany.

0024-4937/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2008.01.004

1. Introduction

Mantle-derived magmas in convergent tectonic zonesexperience a complex evolution from generation to crystal-lisation at shallow depths or as effusives. Magma ascent,emplacement, mingling/mixing in crustal chambers, assimila-tion, storage and recharge take between hours and millions ofyears. Volcanic studies have long made important contributionsto our understanding of the time-scales and rates of magmatic

http://dx.doi.org/10.1016/j.lithos.2008.01.004

406 I. Peytcheva et al. / Lithos 104 (2008) 405–427

processes (see summaries of Hawkesworth et al., 2000, 2004).Such studies usually concentrate on young or active volcaniccomplexes, and employ two main approaches to decipher thetiming of events: (i) absolute dating of different minerals (mainlyzircon), using U-series isotopes (e.g. Reid et al., 1997; Bourdonet al., 2000; Miller andWooden, 2004; Charlier et al., 2005), and(ii) relative dating, based on crystal size distribution (e.g.Cashman, 1990; Marsh, 1998), and/or major and trace-elementand Sr isotope profiles in crystals (e.g. Hawkesworth et al.,2000; Knesel et al., 1999; Costa et al., 2003; Nédli and Tóth,2003.).

Volcanic rocks provide only indirect information about themagma chamber processes in convergent tectonic zones, andtherefore a complimentary record of magma evolution fromplutonic rocks is also necessary. Numerous detailed studiesabout the kinetics of magma mingling/mixing (e.g. Pitcher,1993; Coleman et al., 1995; Castro et al., 1995; Wiebe, 1996;Wiebe and Collins, 1998; Wiebe et al., 2004) and the petrologyand geochemistry of the hybrid intrusives (e.g. Didier andBarbarin, 1991; Bonin, 2004; Janoušek et al., 2004; Peruginiet al., 2004; Annen et al., 2006) have contributed to ourunderstanding of the chemical and physical changes occurringin magma chambers. However, detailed studies addressing thechemical/physical evolution of the magma within an absolutetime-frame are still sparse (e.g. Griffin et al., 2002; Kempet al., 2005), hampered mainly by the precision of the availableisotope methods. Upper to middle crustal magma chambers(5–12 km deep) usually take millions of years before beingexposed at the surface. Therefore, intrusive rocks are usuallytoo old to be dated by U-series isotope methods, requiringother methods that have larger absolute uncertainties. Con-ventional Sm/Nd and Rb/Sr isotope methods are helpful asgeochronometers only in the case of isotope homogenisation ofthe whole rock (e.g. Pin et al., 1990; Didier and Barbarin,1991), which is more an exception than a rule in mixed/mingled magmas. Ar/Ar and Rb/Sr mineral dating provideinformation about cooling time of the magmatic complex. Wechose the U–Pb single zircon ID-TIMS (Isotope Dilution —Thermal Ionization Mass Spectrometry) technique because it isthe most precise method for dating plutonic rocks. Whencombined with Scanning Electron Microscopy-Cathodolumi-nescence (SEM-CL) imaging and in situ laser ablation (LA) ICP-MS analyses, it is a powerful tool to understand the chemical andthermal evolution of the long-lived magma chambers. Zircon ispreferred also because its behaviour during different geologicalprocesses is well studied (see summary of Hanchar and Hoskin,2003 and references therein). Though felsic (granitoid) rocks areusually preferred for U–Pb dating because zircon is a commonaccessorymineral, the present study will show that with a carefulselection of the zircons, hybrid gabbros and diorites may providesuitable zircon for dating.

In subduction-related tectonic zones, the primary uppermantle magma is enriched with incompatible elements that arethought to be introduced by slab-released fluids or slab melts.This effect is largely overprinted by subsequent crustalinteraction, when differing volumes of crustal materials mixwith the mantle-derived magma to produce hybrid suites. The

high field strength elements (HFSE), Hf, Zr, Nb, Ta, and Ti havebeen shown to be crucial for seeing through slab influence intothe nature of the pre-subduction mantle wedge, as theseelements remain largely immobile during slab dehydratationreactions. The budget of HFSE and Lu is only affected duringslab melting of the subducted oceanic crust (Münker et al.,2004). Of the HFSE, hafnium is unique in combining thecharacteristics of HFSE chemistry with a powerful isotopictracer (Woodhead et al., 2001). Here we combine U–Pb and Hf-zircon isotope investigations linking the Hf-isotope data tomagma chamber processes as dated by U–Pb zircon geochro-nology. Thus, we can resolve the problem of the shallow levelcrustal contamination, as the inherited and the newly saturatedzircons are characterized by different age and Hf-zirconsignature.

The present study also uses Nd and Sr whole-rock isotopes,to assess the geochemical characteristics of the different magmasources and for understanding the degree of mixing andhomogenisation. Additionally the zircon geochemistry iscombined with the whole-rock geochemistry to understandbetter how the changes in magma chemistry, temperature andH2O content affect zircon chemistry and saturation.

The region of investigation – the central parts of theSrednogorie tectonic zone in Bulgaria (Fig. 1) – was chosenbecause it hosts some of Europe's biggest Cu–(Au)-porphyryand epithermal deposits, and it is well known that world classCu–Au porphyry and epithermal type ore deposits are linked tomixed magmatic suites. The mixing can lead to magmaticvolatile saturation, which can potentially trigger volcaniceruptions and/or the formation of magmatic-hydrothermal oredeposits (Keith et al., 1997; Dietrich et al., 1999; Hattori andKeith, 2001; Halter et al., 2004, 2005). In the CentralSrednogorie zone, different levels of the magma chambers areexposed providing good opportunities for studying magmamixing processes, and structural and petrologic studies reveallong-lived magma chambers with time-stratigraphic record ofmagma replenishment (Fig. 1b, c).

2. Geological background and sampling

2.1. Geological setting and sampling

The Apuseni–Banat–Timok–Srednogorie (ABTS) belt(Fig. 1a) is part of the Alpine–Mediterranean mountain system.The belt is Europe's most extensive belt of calcalkalinemagmatism and Cu–Au mineralization and resulted fromsubduction and obduction of former Mesozoic ocean basins andfinal collision of Africa with Europe and a number of smallercontinental microplates (Jankovic, 1977; Sandulescu, 1984;Mitchell, 1996; Jankovic, 1997; Berza et al., 1998; Stampfliand Mosar, 1999; Neubauer, 2002). Extensive U–Pb dating ofzircons from subvolcanic intrusions and major plutons in theCentral parts of the Srednogorie tectonic zone (Central Srednog-orie) in Bulgaria revealed a general younging of the magmatismfrom ~92 Ma in the north to ~78 Ma in the south (CapitanDimitrievo pluton, Fig. 1b) (Von Quadt et al., 2005). This ageprogression correlates with a north-to-south geochemical trend of

Fig. 1. a) Tectonic sketch map of the Balkan Peninsula (after Sandulescu, 1984, and Graf, 2001) with the position of Apuseni–Banat–Timok–Srednogorie (ABTS)belt; b) Geological map of the southern parts of Central Srednogorie (after Ivanov et al., 2001); c) Geological map of the study area (after Georgiev and Ivanov, 2003)with sample localities.

407I. Peytcheva et al. / Lithos 104 (2008) 405–427

decreasing crustal input into the mantle-derived magmas (VonQuadt et al., 2005) and is explained as a consequence of slabretreat during oblique subduction (Neubauer, 2002; Lips, 2002;Handler et al., 2004; Von Quadt et al., 2005).

A chain of plutons (Plana, Gutsal, Vurshilo, Elshitsa–Boshulya and Capitan Dimitrievo) is exposed in the southernpart of Central Srednogorie (Boyadjiev and Chipchakova, 1963;Boyadjiev, 1979, 1984; Dabovski, 1988) at the border with theRhodopemassif (Fig. 1b). They intrude the Pre-Upper Cretaceous

basement. Structural and magnetic (accelerator mass spectro-metry) studies revealed that the intrusion of the plutons wascontrolled by a crustal scale dextral strike–slip fault (Ivanov et al.,2001; Georgiev and Ivanov, 2003) — the Iskar–Yavoritsashear zone (IYSZ) (Fig. 1b, c). It can be traced for a distance of80–90 km (Fig. 1b) with a varied width of 0.4–1.0 km. Intensiveductile and rarely brittle–ductile mylonitization affects the rocksalong the shear zone. Minor shear zones are developed paralleland oblique to the main IYSZ (Fig. 1b, c).


Regionally, magma emplacement was accomplished inlaccolith-like chambers (Ivanov et al., 2001; Georgiev andIvanov, 2003; Georgiev and Lazarova, 2003). The plutonsreveal the characteristics of the “layered intrusions” of Wiebe(1996) and Wiebe and Collins (1998) — “interlayered maficand granitic rocks, which preserve a stratigraphic record ofmagma chamber processes that were active during the crystal-lization of the granite intrusions”. The lower parts of the plutonsconsist of “crystal-rich” gabbro–diorite porphyries (Fig. 2a),diorites and granodiorites; the upper portions are “crystal-poor”granites. Between these two parts, thin sheet-like gabbro orgabbro–diorite bodies intruded. Along the bottom of the mafic

Fig. 2. Field relationships of the intrusive rocks in the southern parts of Central Srb) chilled zones and load casts along the bottom of the mafic sheets; c) veins of leucochilled base of the gabbroic layer in the Velichkovo quarry; e) swarms of basic to inteenclaves and swarms of enclaves in the vicinity of Boshulya village.

sheets, chilled zones and load casts (Fig. 2b) are formed. Thebase of the mafic sheet is commonly perforated by flamestructures and veins of leucogranite (Fig. 2c), which provideway-up indicators (Wiebe and Collins, 1998). Downwardgabbroic lobes rarely form at the chilled base (Fig. 2d), whereasabove the mafic sheets swarms of basic to intermediate enclavesare observed (Fig. 2e). Most mafic sheets are subhorizontal,but in the vicinity of Boshulya village they are steeply (70–80°)N–NE-dipping. Highly elongate enclaves and swarms ofenclaves (Fig. 2f) have attitudes that are parallel to themafic sheet.

Mafic feeder dykes cross-cut the mafic–silicic layering.Near Velichkovo village one is subvertical, cross-cuts the

ednogorie. a) crystal phase rich porphyry gabbro–diorites from Vetren quarry;granite, perforating the base of the mafic sheet; d) lobate enclaves related to thermediate enclaves; the level is cross-cut by basic feeder dyke; f) highly elongate


granodiorite part and passes into the gabbroic sheet. In biggeroutcrops we see interlayering of cumulate sheets, mafic andgranitic rocks, and the latter has a variety of enclaves and lenses(globular, irregular; gabbroic to intermediate) and is cross-cutby basaltic dykes. These relationships give evidence for a long-lived magma chamber with magma replenishment and thesequence of deposits should provide a time record of theprocesses in it.

The samples for the present study are selected from differentlevels of the evolving magma chambers. Samples AvQ-029,031, 032 (Elshitsa pluton and subvolcanic rocks; taken close tothe village of Elshitsa; Fig. 1c) as well as ZI6 (Vurshilo pluton)represent the felsic levels. The sampled level of the Vurshilopluton contains sparse mafic magmatic enclaves (MME) that areintermediate in composition. The most spectacular samples aretaken from the mingled/mixed levels: from an outcrop north ofVelichkovo village, where a sharp contact between gabbro(AvQ-018) and granodiorite (AvQ-019) is observed (Fig. 2b)and from a quarry NE of Vetren village with a quitehomogenous layer in-between the basic enclave-rich levels(AvQ-023 and 024, Fig. 2a). Additionally one feeder dyke(AvQ-098) from the last outcrop (Fig. 1c) and one remobilisedleucogranitic vein (AvQ-235, Fig. 2c) from the Velichkovoquarry were sampled for analyses.

2.2. Petrological evidence for mixing and mingling

The petrology of the plutons was well characterized earlierby Boyadjiev and Chipchakova (1963) and Boyadjiev (1962,1979), but recently Georgiev and Lazarova (2003) turnedattention to some specific peculiarities of the intrusions, whichare typical for mingled/mixed magmas. The gabbros anddiorites both reveal signs of magma mixing, even in theoutcrops with sharp contacts between mafic and felsic sheets.Their mineral composition is similar — plagioclase, hornble-nde, clinopyroxene, K-feldspar and quartz, though in the rockswith gabbroic composition (representative samples AvQ-023and AvQ-024 with orthocumulate texture and coarse-grainedhornblende crystals, Fig. 2a) the K-feldspar and quartz areinterstitial, and plagioclase is observed in two generations —An92–88 as inclusions in hornblende and corroded cores in thesecond generation, and An54–32 as zoned crystals. In thehybrid gabbros to diorites (representative sample AvQ-018) theK-feldspar and quartz belong to the main mineral assemblage,whereas clinopyroxene and biotite are minor constituents(Georgiev and Lazarova, 2003). K-feldspars impregnate, resorband replace plagioclase and hornblende crystals. Plagioclase(An50–37) usually displays oscillatory zoning, marking fluctua-tion in the conditions of crystallization. Felsic rocks in thelayered complexes consist of plagioclase (An47–18), K-feldspar,quartz, biotite and rarely hornblende. Plagioclase forms largeeuhedral to subhedral crystals with oscillatory zoning, opticalinhomogeneity and “domain” structure. K-feldspar formsinterstitial anhedral grains. Common accessory minerals in allrock varieties are apatite, zircon, and magnetite± titanite.

Georgiev and Lazarova (2003) used the hornblende thermo-metry (Blundy and Holland, 1990) and the geothermobaro-

metric method of Anderson and Smith (1995) on selectedamphibole–plagioclase pairs in assumed equilibrium to esti-mate P–T conditions of magma crystallization. Their data rangefrom 900–700 °C/7.0–6.5 kbar (in the cores) to 680 °C/2.5–1.8(in the rims) for the hybrid gabbroic rocks (correspondingdepths from 18–19 to 5–8 km). In the hybrid gabbro–dioritesand diorites they calculate 800 °C/5.0–3.5 kbar for the coresand 660 °C/2.6 kbar for the rims, corresponding to depths of 15to 8–9 km. Conditions of 750 °C/3.2 kbar (depths of about9 km) were calculated for the granitic rocks. These data arepartly in line with the P–T constraints of Kamenov et al.(2003b) for the Capitan Dimitrievo pluton (Fig. 1b). There themore acid varieties show the highest calculated values of P andT, which led the authors to suggest an origin of about 15 kmdepth; the segregated granodiorite magma moved later to anupper-crustal chamber at 4–8 km, which was exposed by thesubsequent erosion. The reverse temperature zoning of the coresand rims of the hornblende and plagioclase in CapitanDimitrievo pluton (Kamenov et al., 2003b) suggest repeatedheating and interaction of hot mantle-derived magma with anewly generated crustal magma. Summarizing all available P–Tdata for the southern plutons of Central Srednogorie, we notetwo possible levels of crystallization of plagioclase andhornblende — one is at depths of about 15–18 km, and thesecond one around 4–9 km.

3. Analytical techniques

3.1. Major and trace elements in rocks

Routine analyses of major elements in whole rocks werecarried out on Li-tetraborate pellets using X-ray fluorescence(XRF) method. Trace element and REE determinations weremade with Laser Ablation Inductively Coupled Plasma-MassSpectrometry (LA-ICPMS) on the same Li-tetraborate pellets.LA-ICPMS spot analyses were done using an Excimer laser(ArF 193 nm) with a gas mixture containing 5% fluorine in Arwith small amounts of He and Ne, connected to a PE SCIEXElan 6000 ICP-MS. For the present study we used a spotdiameter of 40 μm. The reproducibility of the data during thiswork was estimated measuring the NIST 610 standard. Forfurther details the reader is referred to Günther et al. (2001) andHalter et al. (2004).

3.2. Cathodoluminescence (CL) imaging

CL images were made for the studied zircons, which areimbedded in an epoxy-resin pellet and then polished to themiddle of the grains. The CL images were taken from a splitscreen on a CamScan CS 4 scanning electron microscope(SEM) at ETH-Zurich. The SEM is equipped with an ellipsoidalmirror located close to the sample within the vacuum chamberand can be adjusted by electro-motors. The sample can belocated in one focal point while the second focal point liesoutside the sample chamber. Here, the CL light enters the highlysensitive photomultiplier through a quartz glass-vacuumwindow and a light channel. The signal of the photo multiplier

Table 1Major and trace-element composition of representative samples from the southern plutons of Central Srednogorie

AvQ-018 AvQ-023 AvQ-024 AvQ-098 AvQ-019 AvQ-029 AvQ-031 AvQ-032 ZI6 Guz1 AvQ-235

Hybrid Gbd Hybrid Gb Hybrid Gb Microgb Grd Gr Grd-porph Grd-porph Gr Grd Grd

SiO2 53.69 51.20 50.91 50.74 69.96 70.30 70.40 69.68 68.50 63.30 60.08TiO2 0.58 0.59 0.52 0.76 0.36 0.24 0.25 0.27 0.22 0.36 0.47Al2O3 14.60 11.47 15.45 17.88 15.66 13.80 17.11 17.59 16.61 18.64 17.75FeO 8.65 8.05 6.52 9.82 3.67 2.99 2.04 2.58 2.01 3.71 5.49MnO 0.15 0.15 0.13 0.26 0.07 0.14 0.06 0.06 0.08 0.14 0.08MgO 8.16 11.97 9.44 5.22 1.38 0.69 0.87 2.44 0.77 1.31 2.67CaO 11.50 14.46 15.87 11.67 4.47 2.85 2.77 0.37 2.94 5.12 7.44Na2O 1.72 1.20 1.34 2.12 3.30 3.39 3.33 2.91 3.13 4.84 3.62K2O 1.18 0.44 0.27 0.28 2.08 3.13 3.10 4.01 3.56 3.61 1.52P2O5 0.12 0.08 0.07 0.17 0.10 0.08 0.07 0.08 0.09 0.18 0.19LOI 1.32 0.98 0.74 0.39 0.33 0.33 0.71Total 100.35 100.93 101.50 99.67 101.05 98.00 100.00 100.00 98.24 101.55 100.03Sc 53.68 68.29 63.94 36.35 10.40 5.76 6.48 5.53 5.78 6.26 14.33Cr 186.8 742.6 131.9 47.15 26.66 39.34 29.96 24.67 14.08 70.61 31.70Co 34.70 37.66 28.66 29.69 9.30 5.62 4.94 4.43 5.84 7.51 13.87Ni 39.42 111.7 78.39 30.07 5.79 9.76 4.20 6.12 4.44 6.76 3.27Cu 70.22 41.29 69.57 42.61 10.39 28.38 50.27 1064 14.01 19.23 42.90Zn 50.93 47.46 34.77 52.19 25.26 19.89 19.21 206.3 57.25 49.37 30.97Rb 34.76 6.81 4.74 9.46 66.31 105.7 54.33 60.32 114.6 82.40 36.27Sr 388.9 240.2 337.1 713.6 405.4 286.1 373.6 94.96 298.3 1854 575.0Y 14.79 14.43 12.61 19.08 10.12 11.92 11.01 9.31 11.94 21.55 10.60Zr 53.44 41.98 31.06 58.24 95.97 102.1 83.09 90.18 100.1 155.8 107.0Nb 1.59 1.72 1.24 1.72 3.15 4.49 4.65 2.98 5.55 6.52 2.76Mo 0.35 0.44 0.56 4.98 2.35 4.40 0 0.65 1.56 3.78 0.81Cs 0.42 0.19 0.14 0.37 2.14 1.76 0.50 0.52 2.72 1.89 0.550Ba 175.4 96.9 66.3 73.79 520.9 549.8 751.9 518.4 429.8 2162 564. 7La 11.01 5.85 5.25 11.90 22.48 26.54 25.89 19.73 23.29 84.18 18.43Ce 22.82 13.95 11.77 25.86 35.68 44.95 43.92 31.78 39.75 145.4 29.97Pr 2.96 1.98 1.64 3.59 3.67 4.35 4.46 3.39 3.89 15.99 3.250Nd 13.05 9.84 7.96 16.13 13.23 15.73 15.82 11.71 14.14 59.04 11.97Sm 3.11 2.57 2.72 3.52 2.74 2.71 2.36 2.31 2.58 8.11 2.43Eu 1.07 0.80 0.80 1.12 1.08 0.65 0.78 0.71 0.67 2.04 0.78Gd 2.74 2.67 2.22 3.48 1.64 2.12 2.01 1.62 2.18 5.29 2.48Tb 0.48 0.49 0.41 0.55 0.30 0.30 0.34 0.23 0.33 0.67 0.33Dy 2.63 2.86 2.44 3.14 1.71 1.75 1.78 1.52 1.78 3.71 2.023Ho 0.60 0.60 0.59 0.68 0.38 0.38 0.37 0.35 0.42 0.74 0.30Er 1.73 1.63 1.39 1.93 1.04 1.12 1.32 1.01 1.18 2.18 1.32Tm 0.22 0.20 0.20 0.27 0.16 0.23 0.21 0.16 0.21 0.37 0.17Yb 1.45 1.74 1.33 1.76 1.17 1.78 1.59 1.33 1.64 2.54 1.31Lu 0.25 0.22 0.18 0.27 0.17 0.24 0.25 0.19 0.27 0.34 0.15Hf 1.50 1.10 0.83 1.49 2.25 2.54 2.21 2.17 3.01 3.79 2.43Ta 0.08 0.08 0.08 0.10 0.21 0.35 0.37 0.23 0.55 0.53 0.21Pb 5.13 1.50 2.11 4.37 8.36 8.56 25.08 20.59 13.59 46.37 5.05Th 3.32 0.72 0.82 1.98 5.41 12.14 11.25 8.10 11.16 37.34 3.97U 1.34 0.19 0.28 0.55 0.95 2.37 2.88 2.72 1.99 5.13 0.68

Abbreviations: Gb — gabbro; Gbd — gabbrodiorite; Gdr — granodiorite; Gr — granite; porph — porphyry.


is then used to produce the CL picture via a video-amplifier. Ingeneral, weak CL emission (dark colours in the picture) meanshigh amounts of minor and trace elements; strong CL emission(light colours in the picture) means low amounts of minor andtrace elements.

3.3. Laser ablation ICPMS (LA-ICPMS) of zircon

Laser ablation ICP-MS spot analyses were done using thesame Excimer laser (ArF 193 nm) and PE SCIEX Elan 6000ICP-MS, as for the trace elements of the rock samples. Thepellet with the zircons (after the CL imaging) was placed in aclosed cell together with the standard material (NIST 610), from

which the ablated material is carried out into the ICP-MS by theargon gas stream. We used spot diameters of 40 and 20 μm. Thelaser pulse repetition rate is 10 Hz. The elements have beendetected with 10 ms dwell time and 3 ms quadrupole settlingtime. The measurement efficiency was around 70%. Back-grounds were measured for 30 s and the transient signals fromthe sample material to be analysed were acquired forapproximately 30 s. Calibrations for the zircon analyses werecarried out using NIST 610 glass as an external standard. Limitsof detection are calculated as 3 times the standard deviation ofthe background normalised to the volume of the sample ablated(cps/μg/g). The reproducibility of the data during this work wasestimated by measuring the NIST 610 standard.

Fig. 3. Harker's diagrams for representative samples from the studied area. For sample localities see Fig. 1c. Filled symbols are used for the mafic rock varieties.


3.4. U–Pb isotope analyses of zircons

High-precision “conventional” U–Pb zircon analyses werecarried out on single zircon grains at the Institute of IsotopeGeochemistry and Mineral Resources, ETH-Zurich, using anion counter system (Finnigan MAT 262 thermal ionisationmass-spectrometer). Selected zircons were air-abraded toremove marginal zones with lead loss, rinsed several timeswith distilled water and acetone in an ultrasonic bath andwashed in warm 4 N nitric acid. All single grain zircon sampleswere spiked with a 235U–205Pb mixed tracer. Total blanks wereless than 0.002 ng for Pb and U. For further details see VonQuadt et al. (2002). The PBDAT and ISOPLOT programs ofLudwig (1988, 2003) were used for calculating the uncertaintiesand correlations of U/Pb ratios. The calculation of the U/Pbratios include uncertainties of the spike calibration, Pb blankmeasurements, common Pb correction, U and Pb fractionationand U decay constant. All uncertainties are included in the errorpropagation for each individual analysis. The decay constants ofSteiger and Jäger (1977) were used for age calculations, andcorrections for common Pb were made using the Stacey and

Kramers (1975) values. The reported “Concordia age” in thefigures as well as in the paper is referred to the calculationprogram Isoplot of Ludwig (2001, 2003); the 206Pb/238U meanage is based on the average of the selected analyses and thenumber of analyses is taken into account for the error calculation.

3.5. Hf-isotope analyses of zircons

Hf-isotope ratios in zircons were measured on a NuInstruments multiple collector inductively coupled plasmamass spectrometer (MC-ICPMS; David et al., 2001) at theInstitute of Isotope Geochemistry and Mineral Resources, ETH-Zurich. High amounts of Zr in the samples did not create asignificant matrix effect, and this was tested by repeatedanalyses of Zr-doped standard solutions. Measured 176Hf/177Hfwas corrected for instrumental mass fractionation using a179Hf/177Hf=0.7325 (exponential law for mass bias correction).Repeated analysis of the JMC-475 standard solution during themeasurement session yielded a 176Hf/177Hf ratio of 0.282141±14 (2 σ). For the calculation of the εHf values the followingpresent-day ratios of the Chondritic Uniform Reservoir (CHUR)

Fig. 5. Geochemical discrimination diagrams, Rb/30-Hf-Ta⁎3 (Harris et al., 1986) and Yb–Ta (Pearce et al., 1984). Abbreviations: WPG — within plate granites,COLG — collision granites, VAG — volcanic arc granites, ORG — ocean ridge granites.

Fig. 4. Spider diagrams for representative samples from the southern parts of Central Srednogorie: a) Trace-element patterns normalized to N-MORB; b) Chondrite-normalized REE patterns (CI chondrite and N-MORB values after McDonough and Sun, 1995).


Fig. 6. a) Sr/Y vs. Y plot (fields from Defant et al., 1991) illustrating the normalisland arc affinity of the igneous rocks from the southern parts of CentralSrednogorie; b) Th/Yb vs. Ba/La and (c) Th/Yb vs. Sr/Nd plots providing evidencefor enrichment of Th, Ba and Sr from both “slab derived fluids” and “sediments”(Woodhead et al., 2001). Filled symbols are used for the mafic rocks.


were used: (176Hf/177Hf)CH=0.282769 (Nowell et al., 1998)and (176Lu/177Hf)CH=0.0334. For age correction of the Hf-isotope ratios at 82–86 Ma, a 176Lu/177Hf ratio of 0.0050 wasused for all zircons.

3.6. Rb–Sr and Sm–Nd whole-rock isotope analyses

The isotopic composition of Sr and Nd and the determinationof Rb, Sr, Sm and Nd contents were performed at ETH-Zurichusing ID-TIMS techniques. Nd isotopic ratios were normalizedto 146Nd/144Nd=0.7219. Analytical reproducibility was esti-mated by periodically measuring the La Jolla standard (Nd) aswell as the NBS 987 (Sr). The mean of 12 runs during this workwas 143Nd/144Nd=0.511841±0.000007 and 10 runs of the NBS987 standard show an 87Sr/86Sr ratio of 0.710235±0.000012.

4. Major and trace-element rock chemistry

Compositionally the southern plutons of Central Srednogoriezone vary from gabbros to granites with SiO2 content rangingfrom 50.8 to 70.4 wt.% (Table 1). The main geochemicalfeatures of the southern plutons have been reviewed byKamenov et al. (2003a), Georgiev (2004) and Von Quadt et al.(2005). In Table 1 representative analyses mainly for the rockvarieties of the Elshitsa–Boshulya pluton and the nearbyoutcrops of Vurshilo and Gutsal plutons are shown, which arethe focus of the present study. Harker's plots (Fig. 3) displaygenerally negative correlation of SiO2 with TiO2, FeOt, CaO andMgO and positive correlations with K2O, Na2O. However, theconsiderable variation of FeOt, CaO, MgO, K2O, Na2O, andAl2O3 in the rock varieties with similar SiO2 content suggest acomplex evolution of the magma (including assimilation andmixing), rather than simple fractional crystallization. There is apronounced composition gap between SiO2≈ 55–63%.Although former authors described having found rocks in thismissing interval (Boyadjiev, 1962; Boyadjiev and Chipchakova,1963), the intermediate rocks are not widespread in the southernoutcrops of Central Srednogorie. This could be a result of thedifferent erosion level or the lack of full homogenization of themixed gabbroic and granitoid magmas.

The MORB normalized trace-element patterns indicate anenrichment of LILE (large ion light elements), which issignificant only in the intermediate and felsic rocks (Fig. 4).The high field strength element content (HFSE, such as Ce, Zr,P and Hf) is slightly negative compared to the MORB values,but a strong negative Nb anomaly is characteristic for all studiedsamples. These features are typical for subduction-relatedmagmatic sequences.

All basic rocks reveal flat or slightly enriched (relative tochondrite) REE patterns (Fig. 4b). Intermediate and acid rockshave fractionated LREE and relatively flat HREE patterns, astypically found in subduction-related volcanic suites. The REE-distribution in the hybrid gabbro–diorite AvQ-018 and theconduit dyke AvQ-098 lie between the trends of the acid andbasic rocks (Fig. 4b). The degree of enrichment of the LREErelative to HREE measured by the ratio Lan/Ybn is 2.3–5.1 inthe basic rocks and 9.5–22.2 (but mainly 10–12) in the

Table 2U–Pb ID-TIMS zircon isotope data for representative samples from the southern plutons of Central Srednogorie

N AnalyticalN

Sizefraction, μm

Weightin mg

# U ppm Pbppm

206Pb/204Pb 206Pb/238U 2σ error%

207Pb/235U 2σ error%

207Pb/206Pb 2σ error%

206Pb/238U 207Pb/235U 207Pb/206Pb Rho

apparent ages

AvQ-018 (Velichkovo hybrid gabbro) brown zircons1 2402 − 125 + 100 0.0436 prism. 988 14.9 2105.8 0.010600 0.50 0.069483 0.53 0.047542 0.18 67.97 68.21 76.52 0.942 2404 − 125 + 100 0.0244 prism. 617 11.4 579.6 0.012585 0.55 0.082459 0.81 0.047522 0.58 80.62 80.45 75.52 0.703 2405 − 125 + 100 0.0277 prism. 1641 29.5 4609 0.012818 0.49 0.084244 0.50 0.047665 0.12 82.11 82.12 82.69 0.974 2407 − 125 + 100 0.0282 prism. 889 14.6 874.3 0.012835 0.46 0.084237 0.49 0.047601 0.16 82.21 82.12 79.46 0.955 2406 − 125 + 100 0.0339 prism. 1134 21.7 2436 0.011997 0.69 0.078691 0.87 0.047573 0.52 76.88 76.91 78.10 0.786 2436 − 100 + 75 0.0040 prism 3519 55.6 2885 0.013222 0.57 0.087414 0.64 0.047949 0.27 84.68 85.09 96.78 0.90

abr7 2437 − 100 + 75 0.0040 prism 4416 70.8 2182 0.013338 0.57 0.088253 0.66 0.047987 0.32 85.42 85.88 98.63 0.87

abr

AvQ-018 (Velichkovo hybrid gabbro) milky zircons8 IP60 − 100 + 75 0.0234 long prism 270.4 4.87 1367.0 0.013327 1.07 0.087948 1.45 0.047862 0.93 85.34 85.59 92.45 0.77

abr9 IP61 − 100 + 75 0.0218 short prism 306.6 6.22 4500 0.013146 0.62 0.086502 0.72 0.047724 0.36 84.19 84.24 85.60 0.86

abr10 IP62 − 100 + 75 0.0046 short prism 348.7 6.55 692.3 0.013330 0.49 0.089022 1.69 0.048434 1.53 85.37 86.59 120.52 0.45

abr

AvQ-018 (Velichkovo hybrid gabbro) sparkle, transparent11 2432 − 100 + 75 0.0059 prism 264.8 17.2 1488 0.064587 2.45 0.499752 2.61 0.056118 0.85 403.46 411.53 457.06 0.9512 2433 − 100 + 75 0.0099 prism 172.4 10.45 2142 0.062935 0.46 0.482150 0.58 0.055563 0.34 393.45 399.55 434.95 0.8113 2434 − 100 + 75 0.0080 prism 34.02 2.51 356.72 0.065627 0.94 0.501842 1.27 0.055766 0.82 407.58 412.95 443.06 0.7714 2435 − 100 + 75 0.0056 prism 24.40 1.78 263.79 0.062333 0.67 0.488881 2.12 0.056883 1.88 389.80 404.15 487.00 0.49

AvQ-019 (Velichkovo granodiorite) zircons1 IP84 − 100 + 75 0.0090 long prism. 161.1 2.83 332.71 0.013195 0.49 0.087507 1.96 0.048098 1.80 84.51 85.18 104.07 0.43

beige2 IP85 − 100 + 75 0.0088 long prism 69.95 1.28 237.67 0.013248 0.58 0.088579 3.19 0.048490 2.98 84.84 86.18 123.44 0.44

beige3 IP101 − 125 + 63 0.0116 prism 117.3 24.41 4233.1 0.201029 0.66 3.06144 0.74 0.110450 0.34 1181 1423 1806 0.89

abr4 IP102 − 125 + 63 0.0086 prism 106.92 2.00 185.07 0.012575 0.78 0.086639 5.33 0.049969 5.04 80.56 84.37 193.53 0.43

abr5 IP167 − 125 + 63 0.0042 prism 40.62 4.33 98.30 0.059221 0.52 0.48769 2.06 0.59726 1.88 370.9 403.3 593.7 0.47

abr6 IP168 − 125 + 63 0.0054 2 prism 126.25 3.10 103.89 0.013336 0.52 0.092225 2.58 0.050155 2.39 85.40 89.57 202.2 0.44

abr

AvQ-023 (Vetrensko Gradishte hybrid gabbro)1 IP97 − 250 + 200 0.0240 prism. abr. transp 232.25 3.90 1244.9 0.013239 0.53 0.086976 0.76 0.047647 0.52 84.78 84.68 81.80 0.72

pile beige2 IP98 − 250 + 200 0.0189 prism. abr. transp 217.93 5.59 108.97 0.013243 0.60 0.087787 4.00 0.048078 3.78 84.81 85.44 103.13 0.44

pile beige3 IP99 − 250 + 200 0.0189 prism. abr. milky 119.92 2.01 677.45 0.013224 0.49 0.088208 1.77 0.048376 1.62 84.69 85.83 117.69 0.42

muddy pink

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Table 2 (continued)

N AnalyticalN

Sizefraction, μm

Weightin mg

# U ppm Pbppm

206Pb/204Pb 206Pb/238U 2σ error%

207Pb/235U 2σ error%

207Pb/206Pb 2σ error%

206Pb/238U 207Pb/235U 207Pb/206Pb Rho

apparent ages

4 IP100 − 250 + 200 0.0132 prism. abr. milky 74.02 1.41 218.87 0.013231 0.72 0.087592 5.02 0.048015 4.73 84.73 85.26 100.02 0.47muddy pink

5 2469 − 250 + 200 0.0070 prism. abr. milky 236.64 4.00 473.64 0.012570 0.50 0.083027 1.45 0.047905 1.29 80.53 80.99 94.58 0.47brown overgrowths

6 2470 − 250 + 200 0.0044 prism. abr. 115.31 2.18 221.81 0.013282 0.65 0.087930 3.17 0.048014 2.94 85.06 85.57 99.97 0.45transp

AvQ-029 (Elshitsa granite)1 IP29 − 100 + 75 0.0033 ≈isometric 121.17 3.74 70.33 0.013475 0.63 0.088747 5.44 0.047767 5.14 86.28 86.33 87.76 0.522 IP33 − 100 + 75 0.0109 prism abr 103.16 2.28 154.84 0.013562 0.58 0.089163 5.39 0.047682 0.51 86.84 86.72 83.50 0.55

pale beige3 IP86 − 100 + 75 0.0080 prism abr 62.68 1.15 284.14 0.013523 1.11 0.089803 2.60 0.048164 2.24 86.59 87.32 107.32 0.52

pale beige4 IP87 − 100 + 75 0.0020 prism abr 162.73 3.20 206.83 0.013552 1.04 0.090282 3.39 0.048316 3.10 86.78 87.77 114.77 0.42

pale beige5 IP89 − 100 + 75 0.0033 prism 189.16 3.61 215.13 0.013162 0.58 0.089170 4.34 0.049136 4.06 84.30 86.73 154.30 0.51

pale beige6 IP90 − 100 + 75 0.0051 prism 166.73 2.87 415.62 0.013179 0.50 0.087906 1.93 0.048376 1.77 84.40 85.55 117.7 0.43

pale beige

ZI-6 (Vurshilo granite)1 2341 − 250 + 200 0.0101 prism 189.8 3.23 304.78 0.012528 0.48 0.082599 0.75 0.047831 0.55 80.24 80.58 90.91 0.68

pile beige2 2342 − 250 + 200 0.010 prism 163.9 2.74 323.46 0.012963 0.47 0.085282 0.79 0.047716 0.61 83.03 83.1 85.25 0.64

pile beige3 2340 − 250 + 200 0.0146 prism 143.7 2.39 400.1 0.012737 0.49 0.084055 1.31 0.047861 1.15 81.59 81.95 92.39 0.49

pile beige4 2345 − 250 + 200 0.0102 prism 156.9 2.41 746.89 0.012657 0.98 0.084988 2.54 0.048699 2.26 81.08 82.82 133.2 0.46

pile beige5 2243 − 125 + 63 0.0105 prism 188.2 2.82 571.3 0.012665 1.77 0.084469 2.27 0.048369 1.36 81.13 82.33 117.4 0.80

abr6 2344 − 125 + 63 0.0115 prism 157.1 4.64 77.21 0.013274 0.65 0.088028 2.7 0.048099 2.45 85.00 85.66 104.2 0.52

abr

AvQ-235 (Velichkovo leucocratic vein)1 3090 − 125 + 75 0.0082 prism 638.5 11.69 1694.7 0.012912 0.67 0.085359 0.91 0.047945 0.60 82.71 83.17 96.54 0.74

beige muddy2 3091 − 125 + 75 0.0064 prism 166.3 2.985 438.8 0.013211 0.46 0.089327 0.74 0.049039 0.56 84.61 86.88 149.7 0.66

beige3 3092 − 125 + 75 0.0084 prism 208.7 3.722 632.4 0.013305 0.47 0.089398 0.53 0.048728 0.23 85.21 86.94 134.8 0.89

beige4 3093 − 125 + 75 0.0032 prism 197.7 3.889 223.5 0.013152 0.46 0.086975 1.2 0.047962 1.11 84.23 84.68 97.42 0.45

beige5 3094 − 125 + 75 0.0554 prism 675.9 12.78 1272.9 0.012605 0.45 0.083303 0.46 0.047931 0.094 80.75 81.25 95.86 0.98

muddy−brw6 3095 − 125 + 75 0.0342 prism 344.2 7.219 366.7 0.013152 0.46 0.086649 0.55 0.047785 0.29 84.22 84.38 88.65 0.84

beige7 3096 − 125 + 75 0.0116 prism 471.1 15.05 3049.8 0.021152 0.55 0.140357 0.78 0.048127 0.55 134.9 133.4 105.5 0.70

beige8 3097 − 125 + 75 0.0066 prism 828.1 16.15 752.2 0.013046 0.62 0.085882 0.65 0.047746 0.16 83.55 83.66 86.71 0.96

beige9 3098 − 125 + 75 0.0128 prism 324.7 5.244 987.5 0.012784 0.97 0.083997 1.04 0.047653 0.35 81.89 81.89 82.12 0.94

muddy-brw

For sample localities see Fig. 1c.#-zircon description abr — abraded, prism — prismatic, transp — transparent; brw — brownish; Rho — correlation coefficient 206Pb/238U–207Pb/235U.

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intermediate and acid rocks. Negative Eu anomalies are notobserved or are very weak, which suggests that plagioclase wasnot fractionated from any of the magma suites. Discriminationdiagrams for Zr/TiO2 vs. Ce/P2O5 (Müller et al., 1992) aresimilar to those in active continental margins (continental arc—CAP field), though the basic rocks lie in the marginal field to thepostcollisional (PAP) field. Discrimination diagrams of Harriset al. (1986; Fig. 5a) and Pearce et al. (1984; Fig. 5b) alsosupport the subduction-related affinity of the studied rocks.

Additional trace-element correlation diagrams can be used toaddress the possible slab fluid/melt contribution. Sr/Y vs. Y(Fig. 6a, Defant et al., 1991), as well as Ba/Th vs. Th diagramsillustrate normal island arc (andesite–dacite–rhyolite, not adaki-tic) affinities (Münker et al., 2004) of the igneous rocks from thesouthern parts of Central Srednogorie. Positive correlation on theTh/Yb vs. Ba/La (Fig. 6b) and Th/Yb vs. Sr/Nd (Fig. 6c) plotsgive evidence for enrichment of Th, Ba and Sr of the initialmagma from both slab derived fluid and sediment components(Woodhead et al., 2001). Melt derived from sedimentary rockswas not likely important, as the Th/Yb ratio is low in the basicrocks and increases only in the granitoids, where it can be easilyexplained by crustal contamination. Noteworthy are the lowvalues of Ba/La and especially Th/Yb in the gabbroic rocks (filledsymbols on Fig. 6b and c), which emphasize the quite primitivecharacter of the initial magma that was slightly enriched only inelements characteristic for slab derived fluids. These data makethe metasomatized mantle wedge the best candidate for theprimary magma source. Somehow the remobilised granitoid vein(AvQ-235)was enrichedwith the same fluid-transported elementsas the gabbroic rocks, which gives evidence for additionaltransport of these elements to the basic varieties in the upper-crustal chamber.

5. U–Pb conventional zircon dating

5.1. Dating of the felsic (granitoid) levels

Inherited zircons (rounded, beige to colourless) prevail in thesample AvQ-029 of the Elshitsa granite. Pale beige long prismaticto almost isometric zircons from the smallest size-fractions werechosen for the dating (Table 2). Four air-abraded zircons areconcordant and define a mean 206Pb/238U age of 86.62±0.28Ma,whereas the non-abraded grains have 206Pb/238U values,corresponding to 84.3–84.4 Ma (Fig. 7a). The calculatedconcordia age (Ludwig, 2001) of the abraded zircons indicatestime of the crystallization of the granite at 86.61±0.31 Ma.

Analyzed zircons from the Vurshilo pluton (ZI-6) are alsopale beige and prismatic. Non-abraded grains (4 zircons) andone of the air-abraded grains spread around an age of 82 Ma(Table 2) and determine a mean 206Pb/238U value correspondingto 81.42±0.43 Ma (Fig. 7b). One abraded zircon is slightlyolder (206Pb/238U age of 85 Ma), whereas the non-abradedzircons define a concordia age of 80.92±0.52 Ma (Fig. 7b).

The granodiorite from Velichkovo quarry (AvQ-019, Figs. 1cand 2b) lies below a gabbroic sheet, but contains mafic magmaenclaves (MME), whichmeans that at a certain time it representedthe felsic upper part of the chamber. The separated zircons reveal

lead inheritance or lead loss (Table 2). Just few of the zircons—pale beige, with sharp edges are without inheritance, whereasthree of them are concordant (or almost concordant) with a mean206Pb/238U age of 84.92±0.26Ma, and two of them (non-abradedgrains) define amean 206Pb/238U age of 84.66±0.17Ma (Fig. 7c).The latter give a concordia age of 84.60±0.32 Ma.

5.2. Dating of the mixed/mingled (diorite–gabbroic) levels

Three types of zircons are distinguished in the hybridgabbro–diorite from the outcrop in the Velichkovo quarry(AvQ-018, Figs. 1c and 2b): brownish (≈65–70%), milky ormuddy rose-beige (≈30–20%) and transparent and vitreous(b10%). The brown colour of the prevailing zircons in thehybrid gabbro may be due to the high U content (N4000 ppm Uis measured in some grains, Table 2). Most of the analyzedzircons reveal lead loss (Fig. 7e) and just two non-abradedgrains are concordant with a mean 206Pb/238U age of 82.16±0.08 Ma (concordia age of 82.16±0.28 Ma, Fig. 7d). This age issupported by the similar upper intercept age (but with larger 2σerror uncertainties) using a discordia line through all non-abraded zircons (81.1+3.7/−2.9 Ma, Fig. 7e), and by the lowerintercept age of 82.31±0.56 Ma (Fig. 7d) using the abradedzircons. The muddy zircons are less rich in U (270–350 ppm)and define a mean 206Pb/238U age of 84.97±0.39 Ma (Fig. 7f).Colourless vitreous zircons from the small size fraction(Table 2) give an upper intercept age of 442.7±8.3 Ma(Fig. 7g) and suggest the metagranitoids (?) of the metamorphicbasement as the mean assimilated source material.

Transparent and muddy prismatic zircons prevail in the hybridgabbros of the almost homogenous mixed layers (samples AvQ-023 and AvQ-024, Figs. 1c and 2a). Five out of these zircons(sample AvQ-023) yield concordant ages (Table 2), with a mean206Pb/238U value of 84.87±0.13 Ma (concordia age of 84.81±0.34 Ma, Fig. 7h). Brown overgrowths have sharp contacts withthe muddy rose or milky cores (Fig. 9) and lead to negligibleyounging.

5.3. Dating of the leucocratic (remobilised) granodioritic veinAvQ-235

Transparent and muddy, grey–brownish prismatic zirconsprevail in this granodioritic sample. As shown on Fig. 7i the firstgroup of zircons reveal an age ~84–84.5 Ma, the second —around 82 Ma. Multiphase zircons lay in-between, whereas theerror ellipses overlap the concordia curve or are very close to it.

6. Hf-isotope and geochemical zircon tracing

6.1. Hf-isotope characteristics of the zircons

Hf-isotope studies are used to provide geochemical informa-tion for the precisely dated zircons. The ε-Hf values arecorrected for the corresponding age, as determined by the U–Pbmethod (Table 3). The gabbroic varieties reveal the mostpositive and mantle dominated ε-Hf (t) values (Fig. 8). InVelichkovo hybrid gabbro–diorite (AvQ-018) they range


from +7.2 to +10.3, and in the Vetren hybrid gabbro (AvQ-023)from +8.0 to +8.6. Concordant zircons from the felsic rocksyield also positive ε-Hf (t) and give evidence for a primary

Fig. 7. U–Pb Concordia diagram for single grain zircon analyses of samples AvQ-02(i; the grey ellipses are used for the beige transparent zircons, and the black for the mucorresponding data points.

mantle source of their magma. The latter was homogenised withcrustal materials prior to the zircon crystallisation, which lead tothe less positive ε-Hf (Fig. 8). The other tendency in the Hf-

9 (a), ZI-6 (b), AvQ-019 (c), AvQ-018 (d, e, f and g), AvQ-023 (h) and AvQ-235ddy beige or grey-brownish zircons). The ε-Hf data of Table 3 are plotted to the

Fig. 7 (continued ).


isotope characteristics is the increase of the mantle componentwith time: some of the 82 Ma old zircons of the Velichkovohybrid gabbro–diorite reveal ε-Hf (t) +10. In the same samplewe observe also larger scattering of the ε-Hf-zircon (Fig. 8),which give evidence for better homogenisation of the magma in

the case of sample AvQ-023 (85 Ma ago) compared with thehybrid gabbro AvQ-018 at about 82 Ma.

6.2. BSE/CL images and trace-element distribution of thezircons

BSE and CL images of the zircons are used to reveal theinternal textures of the zircons from the different rocks. Thestudied zircons show oscillatory and rarely sector zoning(Fig. 9). In the brown zircons of the gabbroic varieties (Av-Q018) oscillatory zoning patterns (OZPs) are usually widelyspaced, which is characteristic for a low degree of zirconsaturation (Vavra, 1990). Light parts in the BSE images/dark inCL (enriched in heavy elements as U and Th) are mostly in theouter parts of the crystals, but sometimes they form sectors (thepyramid one) or domains with an irregular outline. Inclusions ofU/Th minerals (mainly thorite) are also found attached to thesame U/Th-rich zones in zircon. The muddy/milky zircons ofAvQ-018 and AvQ-023 are rich in melt or mineral inclusions ofbasic and acid compositions or contain very small vapourinclusions.

The acid varieties reveal typical CL/BSE images for zirconsin granitoid and intermediate rocks: fine OZPs and rarely sectorzoning.

All Upper Cretaceous zircons yield similar REE distribu-tions. The intermediate and heavy rare earth elements (REE)reveal constant and typical magmatic (Hoskin and Irland, 2000;Belousova et al., 2002) patterns. Positive Ce anomalies arefound in all analyzed grains. Negative Eu anomalies are weak orabsent (Fig. 9), which is in agreement with the REEdistributions of the host rocks and is explained with a lack ofplagioclase fractionation prior to the crystallization in themiddle/upper crust. Characteristic for some brown rims is theappearance of a weak negative Eu anomaly. Brown zircons ofthe hybrid gabbroic rocks are generally slightly richer in REEcompared to the other studied zircons.

7. Sr and Nd whole-rock characteristics

Strontium and neodymium isotope characteristics of thesamples (Table 4) give evidence for mixing of the enrichedmantle magma with at least one type of crustal member(Fig. 10). On the (87Sr/86Sr)i vs. the SiO2 diagram (Fig. 10a)only samples AvQ-018 and ZI6 (with an age of ∼82 Ma) revealclose initial strontium ratios with increasing SiO2. In the otherstudied rocks the initial strontium ratios spread from 0.7047 to0.7060 in the basic/intermediate varieties and generally between0.7052 and 0.7058 in the acid one and give evidence for mixingof mantle and crustal magma. On the SiO2 vs. (

143Nd/144Nd)idiagram (Fig. 10b) two trends are observed again: the first onewith an increase of SiO2 and a decrease of the neodymium ratio,and the second one with quite constant (143Nd/144Nd)i withincreasing SiO2 (samples AvQ-018, ZI6).

Most rock samples lie on the mantle array of the (87Sr/86Sr)ivs. (143Nd/144Nd)i diagram (Fig. 10c) and follow the trend toEM1 field of Zindler and Hart (1986). One sample (AvQ-023)deviates to the EM2 field. This could be due to primary source

Table 3Hf-isotope data for zircons of the southern plutons of Central Srednogorie (numbers as in Table 2)

Lab. N Sample N Age 176Hf/177Hf 2σ error Epsilon Hf today Epsilon Hf T 82–86 Epsilon Hf T 440

2343 ZI-6 82 0.282855 0.000007 2.94 4.462340 ZI-6 82 0.282845 0.000005 2.58 4.112342 ZI-6 82 0.282888 0.000007 4.10 5.632444 ZI-6 82 0.282845 0.000005 2.58 4.11IP168 AvQ-019 85 0.282973 0.000011 7.11 8.69IP85 AvQ-019 85 0.282864 0.000013 3.25 4.78IP101 AvQ-019 N1800 0.281926 0.000021 −29.9 −28.4IP90 AvQ-029 86 0.282917 0.000006 5.13 6.73IP33 AvQ-029 86 0.282901 0.000006 4.56 6.16IP86 AvQ-029 86 0.282951 0.000002 6.33 7.93IP87 AvQ-029 86 0.282912 0.000011 4.95 6.55IP99 AvQ-023 85 0.282968 0.000004 6.93 8.52IP100 AvQ-023 85 0.282974 0.000003 7.14 8.67IP98 AvQ-023 85 0.282975 0.000004 7.18 8.76IP97 AvQ-023 85 0.282959 0.000002 6.61 8.202435 AvQ-018 440 0.282526 0.000012 −8.70 −7.17 −0.482434 AvQ-018 440 0.282468 0.000007 −10.75 −9.23 −2.542433 AvQ-018 440 0.282495 0.000040 −9.80 −8.27 −1.582403 AvQ-018 82 0.283021 0.000004 8.81 10.32407 AvQ-018 82 0.282940 0.000002 5.94 7.472406 AvQ-018 82 0.283011 0.000005 8.45 9.982404 AvQ-018 82 0.282985 0.000006 7.53 9.062437 AvQ-018 82 0.283009 0.000010 8.38 9.912436 AvQ-018 82 0.282969 0.000002 6.97 8.493090 AvQ-235 (82?) 85 0.282877 0.000008 3.71 5.303091 AvQ-235 85 0.282903 0.000008 4.63 6.223092 AvQ-235 85 0.282876 0.000011 3.68 5.263095 AvQ-235 85 0.282848 0.000003 2.95 4.27

During analysis the 176Hf/177Hf ratio (JMC 475 standard) of 0.282141±14 (2 sigma) is measured using the 179Hf/177Hf=0.7325 ratio for normalization; For thecalculation of the ε-Hf values the present-day ratios (176Hf/177Hf)CH=0.282769 and (

176Lu/177Hf)CH=0.0334 are used, and for 82–86Ma a 176Lu/177Hf ratio of 0.0050for all zircons was taken into account.

Fig. 8. Variations of ε-Hf (t) in Upper Cretaceous zircons from the studied rocksamples. Abbreviation “b” is used for the brown/brownish zircons.


differences (EM2 type of enriched mantle source) or to mixingwith radiogenic crustal rocks at middle/upper-crustal levels.

8. Discussion of the results

8.1. Timing of magma chamber replenishment

U–Pb ID-TIMS analyses of zircons from Upper Cretaceousintrusive rocks in the southern parts of Central Srednogorieshow that the method can be applied to dating replenishmentprocesses in long-lived (more than 1 Ma) magma chambers,when the timing is combined with precise field and petrographicstudies and followed by careful selection and CL/BSE imagingof zircons. The precision of the conventional ID-TIMS analysesduring this study (the majority of measurements were carriedout in the period 2002–2004) was usually less than 0.3 Ma(0.35% 2σ uncertainties) or better to 0.08 Ma (0.1%), when theweighted mean 206Pb/238U value was used for age calculations.Further developments in ID-TIMS dating, such as double Pband U spikes (Amelin and Davis, 2006; Von Quadt et al., 2007),smaller decay constant errors (Schoene et al., 2006), blank andspike calibration in combination with the improved chemicalpreparation of the zircons (lower laboratory blanks andchemical abrasion (Mattinson, 2005) will allow lowering theuncertainties to b0.1% and distinction of geological processesless than 0.1 Ma. Apart from analytical uncertainties a possible

limitation for the accuracy of the dates, in terms of theirinterpretation as magma crystallization ages, can be related tothe possibility that magma chamber replenishment and mixingcould lead to protracted or repeated growth of zircon.

In the Elshitsa–Boshulya and Vurshilo plutons our calcu-lated ages for concordant zircons are concentrated around 86.5–86, 85–84.5 and 82 Ma (Fig. 11). This has to be interpreted as a

Fig. 9. Back scattered electron (BSE) and cathode-luminescent (CL) pictures of zircons from samples AvQ-018 (a) and 023 (b) with the corresponding chondrite-normalized distribution of the REE. Circles on the pictures correspond to the laser ablation spot in the zircon crystals; c) chondrite-normalized distribution of the REEin brown and milky zircons of samples AvQ-018. Abbreviation “b” is used for the brown zircons, “m” — for milky zircons.


series of events at a certain time, as opposed to continual zircongrowth, in at least two magma chambers — one in the northernpart (the region around Elshitsa), and one in the southern part

(Boshulya, Velichkovo and Vurshilo, Fig. 12). Anotherobservation is that even at about 82 Ma (AvQ-018) the fieldrelationships (load casts, flame structures, formation of globular

Table 4Rb–Sr and Sm–Nd isotope data for whole-rock samples from the southern plutons of Central Srednogorie

Nr Rocktype

Rb Sr 87Rb/86Sr 87Sr/86Sr 2σ (87Sr/86Sr)i Sm Nd 147Sm/144Nd 143Nd/144Nd 2σ (143Nd/144Nd) ε-Nd

(ppm) (ppm) error (ppm) (ppm) error T=85 T=85

AvQ-018 Hybrid Gb 63.2 299 0.6119 0.704722 0.000015 0.704703 15.9 67.7 0.1419 0.512625 0.000034 0.512546 0.34AvQ-019 Gdr 58.8 561 0.3032 0.705289 0.000016 0.705224 7.59 40.8 0.1122 0.512542 0.000025 0.512480 −0.96AvQ-023 Hybrid Gb 27.6 305 0.2613 0.705730 0.000011 0.705415 8.00 31.3 0.1541 0.512651 0.000009 0.512565 0.72AvQ-024 Hybrid Gb 4.74 337 0.0407 0.704108 0.000012 0.704059 6.22 23.4 0.1603 0.512648 0.000007 0.512559 0.59AvQ-029 Gr 83.2 289 0.8324 0.706207 0.000020 0.705237AvQ-031 Q-Gdr

-porphyry49.8 324 0.4441 0.706364 0.000012 0.705847 25.3 99.3 0.1538 0.51248 0.000042 0.512394 −2.62

AvQ-032 Subvolcdacite

64.6 77.7 2.4060 0.708149 0.000025 0.705346 2.31 11.7 0.1243 0.512584 0.000016 0.512515 −0.27

ZI-6 Gr 93.8 337 0.7825 0.704996 0.000011 0.704084 16.4 86.7 0.1138 0.512611 0.000009 0.512548 0.37AvQ-098 Microdiorite 6.09 437 0.0403 0.705322 0.000017 0.705273

Abbreviations: Gb — gabbro; Gdr — granodiorite; Gr — granite; subvolc — subvolcanic.


enclaves, Fig. 2) indicate interaction of the gabbroic magmawith not fully solidified, but considerably colder (chilledmargins, Fig. 2b) granitic mush. This observation can beexplained with heat flow through the regular hot basic magmareplenishment. Nevertheless, zircons in the granodiorite AvQ-019 are dated at 84.60±0.32 Ma, and therefore no new zircongrowth was detected. This means that the temperature of themush was kept below the conditions that allow zirconiummobility and new zircon saturation (as overgrowths or newcrystals). One exception is the leucocratic vein of remobilizedgranitoid material (AvQ-235, Figs. 2c and 7e), where we havenew growth of zircon at about 82 Ma, contemporaneous withthe intrusion of the gabbroic magma (AvQ-018, brown zircons).The main population of zircons reveal the age of thegranodiorites (AvQ-019) of 84.56±0.23 Ma, which we interpretas inherited from the granodiorite magma mush.

Usually isotope geochronologists have problems findingzircons in gabbroic rocks. Our studies in these mixed secessionsrevealed hybrid gabbros and gabbro–diorites as highly suitablefor dating— they contain different zircon groups that can be usedfor tracing of the processes during the magma ascent to thesurface. One dyke sample (AvQ-098) is an exception and revealsmostly zircon inheritance. Intermediate varieties are also suitablefor dating, because of the prevailing amount of newly saturatedzircons and general lack of inherited grains (cores). Granitoidrocks in the southern parts of Central Srednogorie showed mostcomplicated batch of zircons. There, most of the studied samplescontain significant amounts of inherited grains or cores, and thesubvolcanic varieties may lack newly saturated Upper Cretaceouscrystals (an example is the granodiorite–porphyry from theVlaykov Vruh deposit AvQ-031, Peytcheva et al., 2003).

8.2. Zircon inheritance, saturation and growth

Placing time constraints on replenishment processes withconventional U–Pb ID-TIMS is possible when each datedmagma has newly saturated zircons roughly representing itstime of crystallization. The high temperature (N1000 °C),primary chemistry and usually high water content in subduc-tion-related magmas favour the lack of inherited zircons

(Watson, 1996). Once the magma is generated, its evolutionfollows different scenarios. Although there are examples wheremantle xenoliths are transported to the surface in hours (Nédliand Tóth, 2007), in convergent tectonic zones the magmausually travels much slower — 10–200 thousands years toN1 Ma (Hawkesworth et al., 2000, 2004; Charlier et al., 2005).Zircon inheritance and saturation are expected to respond toscenarios where after its generation in the upper mantle themagma interacts with crustal materials at different levels — onthe mantle/crustal boundary (Leeman, 1983; Hildreth andMoorbath, 1988; Winter, 2001; Richards, 2003), in the lower-middle crust (Annen et al., 2006), mixing and crystallizingfinally in upper-crustal chambers or being erupted as effusives.

Where and when does zircon saturation start and how longdoes it last? Zircon saturation is most sensitive to temperatureand zirconium concentration of the magma, and less sensitive tothe whole-rock chemistry (specially the M=(Na+K+2⁎Ca)/(Al⁎Si) parameter) and H2O-content (Watson and Harrison,1983; Harrison and Watson, 1983; Miller et al., 2003). In theprimary mantle magma zirconium represents an incompatibleelement. Its concentration is usually low (b70 ppm), and thehigh temperature (N1000 °C) will not allow zircon saturationneither in the metasomatized mantle wedge nor in the lowercrust. Consequently zircon is expected to start crystallizing aftera drastic change in the temperature or in the zirconiumconcentration, or to a lesser extent in the chemistry (M value)or water content. The use of the Watson and Harrison (1983)formula is previously restricted by the range of M valuebetween 0.9 and 1.7 and later extrapolated to 1.9–2.1(Hansmann and Oberli, 1991; Von Blankenburg, 1992; Hancharand Watson, 2003). Granitoid rocks match this requirement,revealing usual M values ~1.3–1.5 (Watson and Harrison,1983; Miller et al., 2003). In the southern parts of Cen-tral Srednogorie only acid intrusives are within or close to thisM-value range (between 0.72 and 2.24) and a zircon saturationtemperature of 710–780 °C (mainly ~730 °C) is calculated forthem. These temperatures for the Central Srednogorie granitoidscorrespond to “cold granites” (saturation temperature b800 °C)in the sense of Miller et al. (2003). On the other hand, accordingto their generation they trend to the “hot granites”, as they

Fig. 10. Sr and Nd isotope data for whole-rock samples from the studied area:a) (87Sr/86Sr)i vs. SiO2 and (b) (143Nd/144Nd)i vs. SiO2 diagrams; c) (87Sr/86Sr)ivs. (143Nd/144Nd)i diagram. Fields of the DMM (depleted MORB mantle),HIMU (magma source having a high μ −238U/204Pb ratio), EM1 and 2 (enrichedmantle 1 and 2) and BSE (bulk silicate earth) are given according to Zindler andHart (1986) and Hart and Zindler (1989) (corrected to 85 Ma). Dashed linescorrespond to the Sr and Nd isotope values of the undifferentiated reservoir –identical to CHUR (DePaolo, 1988; Faure, 2001), corrected to 85 Ma.

Fig. 11. U–Pb zircon ages of the studied samples from plutons in the southernparts of Central Srednogorie. Abbreviation “b” is used for the brown/brownishzircons of sample AvQ-018 and AvQ-235, “m” for the milky zircons of sampleAvQ-018.


crystallize from magma with mantle origin, which wascontaminated with crustal material. Therefore, either thetemperature is underestimated (the case, “when natural sampleand experiments did not agree”, Hanchar and Watson, 2003)

because of zircon inheritance in an otherwise undersaturatedmagma (Miller et al., 2003), or the magma reached the zirconsaturation temperature in the upper-crustal chamber aftercooling and during contemporaneous crystallization with theother rock-forming minerals.

In the hybrid gabbroic samples the M values range from 3.9to 6.0, and the FM value (Na+K+Mg+Fe+2⁎Ca)/(Al⁎Si) ofRyerson andWatson (1987) is between 10 and 23. It is clear thatthe Watson and Harrison (1983) saturation thermometry cannotbe applied. Nevertheless zircons are abundant. Why and whendid they saturate? It does not happen because the zirconiumconcentration was increased (it ranges between 31 and 58 ppmin the basic samples, Table 1) and we have to identify otherimportant variables — temperature, water content and chem-istry (changes inM or FM values). TheM (FM) value cannot becrucial — in the gabbroic rocks it stays out of the “limits” ofHarrison and Watson (1983), so that only the changes in thetemperature and water content (hence the physics of magma)tend to be crucial. Mingling and limited mixing processes withcolder granitoid magma/mush in middle- upper-crustal cham-bers favour both — the sudden and fast cooling of the gabbroicmagma, as well as magma degassing. Both of them can lead tozircon saturation and will best explain the strange features of thezircons in the hybrid gabbros: high U- and REE-content;general lack of Eu-anomaly, as is characteristic for the wholerock; appearance of weak negative Eu-anomaly in some outerzircon rims, crystallizing after a significant amount ofplagioclase; inclusions of potassium feldspar; presence ofmany small (vapour?) inclusions in the milky zircons.

Although new zircon saturation is most important for datingof the South Srednogorie plutons, the zircon inheritance canalso be used to assess processes impacting mafic and felsiccalc–alkaline magmas prior to and during their crystallization incrustal chambers. In the hybrid gabbros and gabbro–diorites ofsamples AvQ-018 and 023 the milky zircons often show brownovergrowths that suggest multiple periods of zircon growthduring the magma mixing. In this case the conventional datingwill result in poorly defined ages while the analysed data pointsspread along the concordia line. Abrasion of the rims leads to

Fig. 12. Schematic model of the evolution of the magma chamber producingBoshulya, Velichkovo and Vurshilo plutons (modified after Wiebe and Collins,1998 and Halter et al., 2005). The first portion of basic magma intrude astratified chamber at ~84.9 Ma (sample AvQ-023), almost contemporary with agranodiorite AvQ-019. Mafic enclaves are formed above the sheet-like hybridgabbroic body. Later, at ~82 Ma a second gabbroic magma enter the chamber(AvQ-018), using conduit dikes. The hot basaltic magma mixes with thegranitoid mush (AvQ-019). Hot deep crustal zones with hydrous basaltic magmagenerate intermediate and silicic igneous rocks through fractional crystallizationand partial melting of pre-existing crustal rocks. Vurshilo granite (ZI6) formsabove the sheet-like body of the hybrid gabbro AvQ-018.


more precise age, but unfortunately removes the younger parts.We could suppose the same phenomena for zircons, where thismultiple growth is not obvious (not marked by clear differencesin the U-content), for example in the zircons of ZI6.

The hybrid gabbro–diorite sample AvQ-018 contains also(sparse) inherited vitreous zircons with ~440 Ma age, whereasin the granitoids the zircon inheritance is poorly defined, orpoints to Proterozoic ages (Table 2). Older U–Pb multigrain agedata on zircons from the basement gneisses (unfortunately withunclear sample localities) of Arnaudov et al. (1989) are difficultto interpret because of the larger uncertainties −406±30, 480±30 and 485±50 Ma. Recently Carrigan et al. (2006) measuredzircon cores in a Hercynian leucosome enclosed in migmaticparagneisses with age populations at ~500–700, 900–1100,1900–2200, 2350, 2550 and 2700 Ma (HR-SIMS analyses).Peytcheva and von Quadt (2004) reported preliminary protolithages of 502.8±3.2 Ma for amphibolite-facies overprinted gneissfrom the northern part of Central Srednogorie and a concordantage of 443.0±1.5 Ma for a low metamorphic metadiabase in theadjacent southern slopes of the Balkan. Inherited zircons andcores with mantle origin from the Medet monzodiorite(Peytcheva et al., 2004, 2006) define a discordia line with anupper intercept age of 456.5±5.5 Ma. The published ages leadto the assumption that the sources for the basic/intermediate andacid varieties of rocks in the southern parts of CentralSrednogorie are quite different. One possible candidate for theinheritance in the basic and intermediate magma may be thelower crust — this would explain the mantle or mixedcharacteristics of the inherited grains and cores. In this casethe geochemical data for the most primitive magmas in presentstudy (AvQ-023, 024 and 018) characterize not the metasoma-tized mantle wedge-derived magma, but rather the products ofits first mixing in a lower chamber (MASH zone?). There thehot magma (N1000 °C) with high M value ((Na+K+2 ⁎ Ca)/(Al ⁎ Si), Watson and Harrison, 1983) will favour the

dissolution of assimilated zircon. Only the fast transport andrelatively fast crystallization will allow them to survive and bepresent in the studied Upper Cretaceous rocks.

According to the zircon inheritance in the felsic samples(AvQ-019, AvQ-029, ZI6) the granitoid magma is a result notonly of differentiation (AFC) processes of enriched mantlemagma, but they also need an additional source within themiddle/upper crust (e.g. Griffin et al., 2002; Yang et al., 2007).Inherited zircons in the studied samples reveal typical crustalgeochemical characteristics (negative ε-Hf (t) of −22 to −28,Table 3, Peytcheva and von Quadt, 2004). Presumably theirpreservation was favoured additionally by the chemistry of themagma (low (Na+K+2 ⁎ Ca)/(Al ⁎ Si) ratio) and the typicallylower H2O content of the crustal magma.

8.3. Isotope-geochemical peculiarities of the mingled/mixedmagma revealed by Hf-zircon and Nd–Sr–WR isotope data andmajor and trace elements

Three possible magma sources for the plutons from southernparts of Central Srednogorie – slab derived LILE and LREEenriched fluids, the metasomatized mantle wedge and thecontinental crust – can be identified by the combination ofwhole-rock trace-element chemistry and Nd–Sr isotope char-acteristics. As mentioned above, indicative ratios such as Sr/Y,Ba/Th, Th/Yb and Ba/La in the basic (most “primitive”) rockspoint to slab derived fluids as an important source for mantlewedge enrichment (Fig. 6b, c). The sediment component isnegligible as we cannot relate the increased Ba/La and Sr/Ndratios to significant crustal-sediment contamination (no changein the Th/Yb ratio, Fig. 6b, c). The intermediate and acid rockvarieties reveal both the slab derived fluids and the sediment asenrichment source, but geochronological data and Hf-isotopestudies of the zircons relate the sediment component to mainlycrust assimilation.

It seems that the mingling/mixing of the gabbroic magma(samples AvQ-018, 023 and 024) with the felsic one (AvQ-019and possibly ZI-6) was either limited or occurred withfractionally crystallized magma with little crustal contamina-tion, or both— even after mixing, the Sr and Nd characteristicsand trace-element signature of the basic hybrid rocks remainquite “primitive”.

The Lu–Hf zircon data are most reliable for tracing theprocesses that affected the basic and acid magma, as they arerelated to the dated zircons. The advantage of the isotope Hf-zircon analyses is the possibility of distinguishing between oldinherited zircons (ε-Hf of −28, Table 3) and newly saturatedgrains (mantle dominated), instead of the commonly used ε-Hfwhole-rock analyses. The ε-Hf values of all concordant zirconsfrom the Upper Cretaceous plutons in Central Srednogorie arepositive (Fig. 8), which was also shown for the subvolcanic andvolcanic varieties in the region in previous studies (Von Quadtet al., 2005). In the hybrid gabbro of Velichkovo (82 Ma), ε-Hfis slightly higher than +10 and in the monzogabbro of CapitanDimitrievo pluton (Kamenov et al., 2003a,b), it is +9.2. Thesevalues are similar to that of the measured concordant UpperCretaceous zircons from andesites (basaltic andesites) in the


Timok region (+9.5 to +13, Von Quadt et al., 2003) and inmonzodiorites of Eastern Srednogorie (Georgiev et al., 2005;Ruskov et al., 2006) of the ABTS belt. The Hf-zircon isotopecharacteristics lead to the assumption that the mantle wedge wasnot enriched with sediment-derived Hf compared with the datafor the Palaeozoic subcontinental mantle (Griffin et al., 2000,2002; Spetsius et al., 2002; Peytcheva and von Quadt, 2004;Yang et al., 2007, and unpublished data). They are generally inthe range of the recent Atlantic MORB Hf-isotope character-istics (+8 to +21, Chauvel and Blichert-Toft, 2001). It is alsopossible that the gabbroic varieties of the Central Srednogorieare themselves a product of at least one mixing/assimilationevent in a lower chamber and consequently the “primitive”mantle wedge magma in Upper Cretaceous time was morepositive in ε-Hf values than the measured values.

The positive ε-Hf values of the Upper Cretaceous zircons inthe granitoid varieties emphasize the role of AFC processes inthe evolution of the magma: even after the assimilation ofcrustal material and the mixing/mingling in the crustal chambersthe age corrected ε-Hf values are mainly from +4 to +8(Table 3). Another tendency of the Hf-isotope characteristics isthe increase of the mantle component with time (Fig. 8): someof the 82 Ma old zircons of the Velichkovo hybrid gabbro–diorite show the most positive ε-Hf (t) +10. Zircons of the samesample reveal a larger scattering of the ε-Hf compared to thedata for the hybrid gabbro of Vetren quarry (85 Ma old). Thedifferent scattering of the ε-Hf zircon data we interpret torepresent a different grade of homogenisation (more thoroughmixing in the chamber at 85 Ma, compared with the mingling/mixing at ~82 Ma).

Present data confirm the observation of Hoskin et al. (2000)about the insignificant changes of REE-distribution in zirconswith progressive differentiation (Fig. 9). However, some strikingpeculiarities are seen in the brown zircons of the hybrid gabbro(AvQ-018): they are generally slightly richer in REE comparedto the other studied zircons and some have a weak negative Euanomaly. Additionally (as observed also by Hoskin et al., 2000)zircons of the hybrid gabbroic rocks reveal the highest U (andTh) contents. Both features are not usual for basic rocks and inferthe special case of hybrid gabbroic magma, in which mixingleads to zircon saturation in shallow chambers and to crystal-lization of the zircons together with or after some rock-formingminerals, thus allowing the zircon to incorporate U–Th and REEelements from the fluid-rich magma.

9. Conclusions

1. The Upper Cretaceous southern plutons of Central Srednog-orie were formed in long-lived magma chambers thatexperienced magma replenishment. Mingling and mixingprocesses of basic and felsic magma are inferred from fieldrelationships and petrographic evidence for the rocks.

2. Conventional ID-TIMS U–Pb zircon geochronology issuitable for dating replenishment processes in long-livedmagma chamber(s) when they are sufficiently separated intime (≥ 0.6–1.0 Myr) and marked by new zircon growth.The precision of the analyses (usually less than 0.35% 2σ

uncertainties) can distinguish a series of zircon crystal-lization ages around 86.5–86, 85–84.5 and 82 Ma. Con-tinuous heat flow provided by periodic gabbroic magmareplenishment may account for the preservation of thegranitoid mush in the chamber for long periods of time andfor granitoid remelting.

3. Major and trace-element distribution, Sr and Nd isotopechemistry, as well as indicative ratios such as Sr/Y, Ba/Th,Th/Yb and Ba/La define subduction-related normal islandarc affinities for the studied plutons. These trace elements areused to identify slab derived fluids as an enrichment sourcefor mantle wedge magma, whereas a sediment component inthe granitoid rocks is due to crustal contamination.

4. Zircon inheritance and the corresponding ε-Hf in the hybridgabbroic rocks point to the lower crust as a possible sourcefor the second enrichment of the primary mantle magma,whereas upper and middle crust contributed significantly inthe generation of the granitoid varieties.

5. Hf-zircon isotope characteristics mark a slight increase of themantle component with time: some of the 82 Ma old zirconsin the hybrid gabbro reveal the highest ε-Hf (t) up to + 10.3.The broad scattering of the ε-Hf-zircon within one samplecould be a result of poor homogenisation of the mantlemagma after the crustal contamination.

6. Abundant zircon found in the mingled/mixed successionsreveals that hybrid gabbros and gabbro–diorites may besuitable for dating. Different zircon groups can be distin-guished within the rocks, which can be used to trace magmasources during ascent of the magma.

7. Mingling/mixing of the mantle magma with cold crustalgranitoid magma/mush at mid- to upper-crustal levels isproposed to explain the change of the physical conditions (fastcooling; magma degassing?) and to lesser extent in the magmachemistry that are necessary for zircon saturation and fastcrystallization of U- and REE-rich zircons in the hybrid gabbro.

Acknowledgments

This work was supported by the Swiss National ScienceFoundation through project 200020-100735 and the SCOPESJoint Research Projects 7BUPJ62396. The study is a contributionto the GEODE-ABCD (Geodynamics and Ore Deposits Evolu-tion of the Alpine–Balkan–Carpathian–Dinaride province) of theEuropean Science Foundation. Discussions with Urs Schaltegger,Peter Marchev and colleagues from IGMR, ETH-Zurichencouraged the authors to publish the data. IP appreciates theEnglish corrections of Svetoslav Georgiev and the editing of therevised version byBlair Schoene, whomade the foreigner Englishreadable. Wendy Bohrson and an anonymous reviewer arethanked for their constructive corrections that helped to improvethe manuscript.

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