The Triassic metabasalts of Dudin Kr { , near Kosovska Mitrovica , Serbia

This paper presents the geochemical characteristics of the metabasalts of Dudin Kr{, near Kosovska Mitrovica. The Dudin Kr{ is the easternmost occurrence of Middle Triassic rift–related volcanic rocks in the Dinarides. Generally, these rocks show similarities to other Triassic volcanic rocks of the Dinarides. They are high–magnesian, ol and ne–normative basalts, with low Zr/Nb and medium/low Ti/Zr and Ti/Y ratios. They exhibit transitional geochemical characteristics, between E–MORB and subalkaline basalts of continental rift zones. Their presence and geochemical affinity is evidence of rifting processes along the continental slope of the Adria block during Middle Triassic.


Introduction
The Mesozoic history of the western parts of the Balkan Peninsula has long been and still is a matter of debate.The study of Middle Triassic volcanic and metavolcanic rocks is very important in elucidating relationships between recently present tectonostratigraphic terrains/units and ancient geotectonic environments.
Middle Triassic igneous rocks of the Dinarides occur along NNW-SSE elongated tectonic lineaments, stretching from the South Alps in the north-west to the Albanides-Hellenides in the south to south-east.It is generally believed that they originated by rift-related processes, which occurred within the Adria plate, as a northern Gondwanaland promontory (DIMITRIJEVI], 1998 and references therein).These rocks are variable in mi-neral composition and geochemistry, but also in their tectonostratigraphic position with respect to the presumed northern margin of the Adria.In this context, from south to north, five subparallel zones of Mid-Triassic extrusive rocks can be distinguished in Serbia and Montenegro (Fig. 1): (1) behind the carbonate platform at the continental margin, now represented by the Budva zone, (2) along fracture zones originating at the continental shelf in the Triassic carbonate platform or in the Paleozoic basement of the External Dinarides (northern Montenegro and further to the north-west), (3) along the main rifting zone (the precursors of the Dinaridic oceanic basin or marginal sea), (4) in the setting of the carbonate platform of the Drina-Ivanjica unit at the northern side of the Dinaridic oceanic basin, and (5) at the continental slope of the Adria block, sit-uated within the Triassic series, now on the western side of the Kopaonik block.These Middle Triassic rocks probably originated within the same geotectonic setting, i.e. they are related to rifting of the marginal parts of the northern Gondwanaland promontory.
This paper is focused on a more detailed geochemical identification of the metabasalts of group 5.These occur near Kosovska Mitrovica as small outcrops of Triassic metavolcanics, tectonically belonging to the northernmost parts of the Adria block.New samples of metabasalts from the locality of Dudin Kr{ are present- ed and discussed.The contents of major and full-range trace element, including rare-earth element (REE) concentrations, were determined.

Geological Setting
The Middle Triassic metabasaltic rocks near Kosovska Mitrovica have a specific tectonostratigraphic position.They belong to the association formed on the continental slope of the Adria block.These rocks occur as lava flows or as dykes or sills, which alternate with silty-argillaceous rocks of the Central Kopaonik series of Middle Triassic (KLISI] et al., 1972) or Carnian (SUDAR, personal communication) age.Presently, they can be found on both sides of the Western Branch of the Vardar suture (zones 4 and 5).
The best outcrops of the seqence are found at the locality of Dudin Kr{ ("Dudas Rock"), at the right side of the river Ibar, along the road Kosovska Mitrovica--Zve~an (Fig. 1, upper right).Other outcrops may be found along the road Kosovska Mitrovica-Trep~a Mine, and some are situated on the northern side of the Jo{anica River, west of Jo{ani~ka Banja.
Metabasalts of Dudin Kr{ occur as flattened pillows (the larger axis ranges from n-dm to 2 m, the shorter one up to 5-6 dm) with well developed zoning.The cores often contain small chloritic amygdales, whereas the rims are compact.The rock mass of the pillows is schistose at the margins and sometimes foliation is present even through the entire body of the pillows.The color of the pillows is green and sometimes reddish along the rims.The pillows are embedded into a chloritic groundmass, containing minor calcite and haematite as main secondary minerals, which developed at the expense of hyaloclastic glass shards.The texture of the rocks is blastoophitic, intersertal to pilotaxitic, with a decrease of needle-and grain-sizes from the core of the pillows towards the almost glassy rims.The studied rocks ubiquitously express the effects of low-to medium-temperature metamorphism and show mineral composition characteristic for spilites.The major mineral constituents are relics of primary clinopyroxene, and an association of albite, chlorite, epidote and pumpellyite.
Concentrations of Th, U, Ta, Hf and the REEs were analyzed by instrumental neutron activation analysis (INAA) at the Vernadsky Institute of Geochemistry and Analytical Chemistry, The Russian Academy of Sciences, Moscow.The samples were irradiated in a "MIFI"-reactor (Moscow), at a neutron flux of 1.2*10 13 m/cm 2 *sec, for 15 hours.Then the samples were "cooled down" for 5-7 days and their activity measured on a multichannel gamma-spectrometer NUC-8192, with a Ge-detector GEM-15180-P ("Ortec"), with resolution on line near 1.7 keV, 60 Co (E i =1332 keV).The spectra obtained were evaluated using the peak-fitting routine of SHUBINA & KOLESOV (1998).The concentrations were calculated by irradiating and counting of standards of basites (ST-1A, SGD-1A, AN-9) under identical conditions.La, Nd, Sm, Eu, Yb, Th, Hf and Ta have better than 5% and Ce, Gd, and Tb have 10% confidence limits.

Major elements
Major element analyses are shown in Table 1.Connected with extensive low/medium-temperature metamorphism of the rocks, the studied samples show increased and variable contents of LOI (7.47-10.37wt%).Nevertheless, there are features of the primary composition of the rocks which are preserved.Accordingly, samples DK-2, DK-3, DK-4 of rocks are olivine-and nepheline-normative basalts characterized by low silica (SiO 2 up to 42 wt%), medium titanium (TiO 2 ~1.6 wt%), low alumina (Al 2 O 3 up to 14 wt%), high magnesium (MgO around 10 wt%), high and variable CaO (9-14.6 wt%), high MnO (~0.25 wt%) and very low K 2 O (0.02-0.05 wt%) contents.The low contents of SiO 2 and Al 2 O 3 , along with the increased contents of MnO are apparently related to pervasive chloritization.

Trace elements and REEs
Trace element analyses and REE concentrations are shown in Tables 2 and 3, respectively.Because of potential mobility of most major and large-ion lithophile elements during post-magmatic low/medium-temperature metamorphic events, the geochemical identification of the studied samples are based on their HFSE, Th and REE contents and ratios.These elements are generally considered as immobile, and their stability during processes of alteration, low/medium and even high-grade metamorphism has been widely demonstrated (e.g.MÖRK & MEARNS, 1988;GRAUCH, 1989;BIENVENU et al., 1990;SHATSKY et al., 1990, etc.).However, the primary contents of these elements in some cases can be disturbed under low temperature conditions in the presence of CO 2 -, F-, or Cl-rich fluids and at high fluid/rock ratios (MURPHY & HYNES, 1986;BIENVENU et al., 1990;VERMA, 1992).The primary contents of HFSE, Th and REE in the studied rocks are likely to have been preserved, which is suggested by: (a) the absence of a correlation of the contents of these elements with variations of LOI, K 2 O, Na 2 O and CaO, (b) the presence of regular variations of the contents of Ti and Zr and (c) rather smooth patterns on multi-element spiderdiagrams (Fig. 2).
The studied samples exhibits high concentrations of Nb, Zr, Sr, Y, P and low Zr/Nb and Ti/Zr ratios of ~8 and ~90, respectively.
The characteristic ratios of REE and Th/REE of the high-magnesian metabasalts (La/Sm) n ~2, (La/Yb) n ~4, (Tb/Yb) n ~1.5, Sm/Nd ~0.3, Th/La = 0.25, Th/Sm = 0.60, Th/Yb = 0.9, demonstrate their average relative enrichment by LREE and Th compared to basalts of normal segments of oceanic spreading zones (N-MORB).The other important features of these metabasalts are: a) the absence of negative geochemical anomalies of Ta, Nb, Zr and Ti (Fig. 2); b) high contents of HREE and Y, (Yb) MORB = (Y) MORB = ~1; and c) that they lie along the mantle correlation array on the Th/Yb-Ta/Yb diagram (Fig. 3).The established geochemical characteristics of the studied metabasalts may infer the nature of their mantle sources and conditions of origin of their primary melt.The major conclusion of the geochemical identification is that they correspond to a transitional type between E-MORB and subalkaline intraplate basalts or subalkaline basalts of continental rift zones (SUN & MCDONOUGH, 1989;HEGNER & PALLISTER, 1989;BARRAT et al., 1993;BRIAND et al., 1995, etc.).Such low Zr/Nb ratios have already been reported in Triassic basalts from the Vare{ area (KA-RAMATA et al., 2000), which are spatially related to Dinaride ophiolites and probably represent relicts of the axial parts of the rift (zone 3 in Fig. 1), nowadays situated at its margin.In all other zones of Triassic rocks, the Zr/Nb ratios are above 10.However, the Dudin kr{ metabasalts show Ti/Zr and Ti/Y ratios similar to those of Triassic basalts in the External Dinarides (BILIK et al., 1993;DJUJI] et al., 1995).The presented data do not show clear evidence of crustal contamination.It is noteworthy that the Th/La, Th/Sm, and Th/Yb) ratios of the Dudin Kr{ metabasalts are slightly higher than those characteristic for basalts of a transitional type.This may be attributed to the involment of continental crustal components, as is typical for the initial stages of the formation of oceanic basins (HEGNER & PALLISTER, 1989;BARRAT et al., 1993;CHAZOT & BERTRAND, 1993;KAZMIN & BYAKOV, 2000, etc.).

Concluding Remarks
The metavolcanic rocks of Dudin Kr{, near Kosovska Mitrovica, belong to rift-related Triassic volcanic rocks, which occur at various places in the Dinarides.Geological observations imply that the studied basaltic rocks were emplaced along the ancient continental slope of the Adria block.These rocks represent the easternmost occurrences of Middle Triassic rift-related rocks in the Dinarides.Strictly speaking, the applied methods of geochemical identification generally fail to unambiguously classify basalts erupted on an attenuated continental crust.Nevertheless, the following characteristics were recognized: they belong to the high-magnesian group, and also display low Zr/Nb and medium/low Ti/Zr and Ti/Y ratios.These rocks show similarities with other occurrences of Triassic volcanic rocks in the Dinarides.The transitional character between E-MORB and subalkaline intraplate basaltic series can be considered as evidence of stretching imposed on the continental slope zone of the Dinarides in the Middle Triassic.

Fig. 1 .
Fig. 1.Geographic and geotectonic position of Dudin Kr{ Triassic metavolcanic rocks.The left sketch displays the major geotectonic units and distribution of Mesozoic ophiolites and Triassic volcanics of the south-western Balkan Peninsula (after DIMITRIJEVI], 1992 and KARAMATA et al., 1999).Simplified geology of the Dudin Kr{ area (after BOGDANOVI] et al., 1977).The zones 1-5 are referred to in the text and they are: 1 = Budva zone; 2 = Northern Montenegro; 3 = Triassic volcanics spatially related to the Dinaridic ophiolite belt; 4 = Triassic volcanics occurring in the Drina-Ivanjica unit; 5 = Triassic volcanics occurring at the western margin of the Kopaonik block.

Fig. 2 .
Fig. 2. Multi-element patterns of Dudin Kr{ metabasalts of the Adria block.The concentrations are normalized with respect to the composition of C1 chondrite and N-MORB from SUN & MCDONOUGH (1989).