PETROLOGY OF THE GARNET AMPHIBOLITES FROM THE TEJI]I VILLAGE (POVLEN MT., WESTERN SERBIA)

Different metamorphic rocks discovered near the village of Teji}i (Povlen Mt., Western Serbia) represent members of the olistostrome mélange metamorphosed during the obduction/emplacement of some still hot ultramafic body. They occupy the area of about 2 km. The garnet amphibolites are of highest metamorphic grade in the area of Teji}i and were chosen as the most convenient rocks for determination the pressure‡temperature conditions of metamorphism and of ultramafics during their emplacement.


INTRODUCTION ‡ GEOLOGICAL SETTING
The village of Teji}i is located in the so ‡called "Mesozoic zone", situated between two large areas: the Drina ‡Ivanjica and the Jadar Paleozoic.This zone was separated by Petkovi} (1930.According to Mojsilovi} et al., (1966Mojsilovi} et al., ( , 1975)), different sedimentary, igneous and metamorphic rocks within this zone, excluding only some large blocks, were regarded as members of the "Diabase ‡Chert Formation", until Dimitrije-vi} & Dimitrijevi} (1974) explained this formation as the ophiolitic mélange.This area is a part of the western subzone of Vardar Zone Composite Terrane (Karamata & Krsti}, 1996;Fig. 1).The Vardar Zone Composite Terrane is in the area of Teji}i only about 5 km in width and thrusted over the Drina ‡Ivanjica Terrane on west, and over Jadar Block/Terrane, on east.It is composed of dismembered ophiolite complex, Triassic, mainly carbonaceous, and Liassic and Cretaceous carbonaceous blocks, and of sandstone blocks as the less abundant, all in clayey-silty matrix.(Kara mata & Krsti}, 1996).In the area of Teji}i the ophiolite complex comprises ultramafic rocks (tectonites), gabbros, diabases (the lowest part of the "sheeted dyke" complex) and metamorphic rocks, that form an discontinuous belt with average thickness of 200 m, locally up to 1000 m.Ultramafic cumulates are rare or completely absent, while the pillow lavas occur separately, further to the north.Metamorphic rocks include amphibolites, various schists and seldom gneisses.Garnet amphibolites were discovered as a few meters thick bodies next to ultramafic rocks, while further from the contact amphibolites without garnet occur.
Garnet amphibolites were chosen for detail investigation, as they represent the rocks of the highest metamorphic grade in this area, i.e. the most convenient rocks for determination the pressure ‡temperature conditions of metamorphism and of the ultramafics during their emplacement.

ANALYTICAL METHODS
Samples were chemically analyzed for major and trace elements by X ‡ray fluorescence, while REE analysis was done by ICP techniques at the Mineralogical Institute in Köln (Germany).FeO in rocks was determined by titration with a standardized potassium permanganate solution at the Aristotle University (Thessaloniki, Greece).
Microprobe analyses are performed by G. Christofides at the Aristotle University (Thessaloniki, Greece), while K/Ar analyses were done by K. Balogh at the Institute for Nuclear Research in Debrecen (Hungary).

PETROGRAPHY AND CHEMISTRY OF MINERALS
Garnet amphibolites are massive, sometimes banded rocks of nematoblastic texture.They are exposed on the southeastern slopes of Kosa next to ultramafic rocks.Their outcrops are about few m 2 and in sharp, i.e. tectonic contact with ultramafic rocks.They consist of amphibole, plagioclase, garnet and epidote, chlorite and prehnite are secondary minerals, while the very seldom rutile is the only accessory constituent.
Amphibole occurs as needles up to 3 mm in diameter, which are gathered in laminaes defining S 1 foliation.Rare occurrences of poorly orientated amphiboles indicate the minor mineral growth after the peak of deformation.Amphibole grains make between 30% and 50% vol. of these rocks.They are nearly homogenous in composition and without regular differences between their core and rim (Table 1).According to Leakes classification (1978) amphiboles correspond to hornblende (Fig. 2).Amphibole grains often enclose garnet and rutile grains.Sometimes they are chloritized on their rims, rarely in their central parts.This chlorite corresponds to corundophillite or to ripidolite (Table 2).
Plagioclase is the second main constituent and build from 30 to 40% vol. of these rocks.Primary plagioclase grains are almost completely transformed to epidote ‡ prehnite aggregates.Their size is about 0.8×1 mm, while the newly formed plagioclases are in irregular grains up to 0.5×0.8mm in size.According to results of microprobe analyses this newly formed plagioclase is almost pure albite with up to 2% of anorthite component (Table 3).Garnet grains are euhedral, up to 0.8 mm in size.According to their chemistry they are almandine with 18-23% of pyrope, up to 13% of grossulare and up to 11% of spessartine component (Table 4).A difference between their core and rim is notably, especially for the MgO content which is regularly higher in the central parts of garnet grains.Epidote and prehnite are of secondary origin, formed after plagioclases.The chemistry of epidote is presented in the Table 5.

GEOCHEMISTRY
Chemical composition of an amphibolite with garnet, and of an amphibolite without garnet, as well as their CIPW ‡norm composition is given in the Table 6.Garnet amphibolite has been analysed, as it has been noted, as the rock of highest metamorphic grade, while the other one (without garnet), has been chosen only for comparation.Their trace elements content, as well as REE concentration is presented on Table 7. Comparation of their chemical composition with chemistry of mafic volcanic rocks, in attempt to identify a possible protolith, indicates that both of amphibolites correspond to subalkaline basalts (Fig. 3).On E ‡MORB normalized diagram garnet ‡amphibolites display a nearly flat pattern and the absence of Eu anomaly indicating on their origin from ocean ridge basalts (Fig. 4).

GEOHRONOLOGY
K/Ar age was determined by K. Balogh (Table 8) using hornblendas from three samples of amphibolites, including the garnet amphibolite.
Ages measured on hornblende concentrates from the garnet amphibolite (T ‡45) and from the amphibole schists (T ‡13b, T ‡24) agree within the limits of experimental errors and give the time when the rocks cooled below the closure temperature of hornblende.According to Harrison & McDougall (1980) the closure temperature of hornblende is 400 ‡540 o C, depending on the grain size and cooling rate.In our case a higher closure temperature of about 500 o C is more acceptable becouse of high cooling rate.Investigated rocks were metamorphosed on the contact with the ophiolite slab and the different degree of metamorphism is a result of different temperature, i.e. distance from the contact.nit (nastali na ra~un plagioklasa; tabela 5), a vrlo redak rutil je akcesorni mineral.

Table 7 .
Content of trace elements and REE in amphibolites.