THE POLUMIR GRANITE ‡ ADDITITIONAL DATA ON ITS ORIGIN

The Polumir granite is exposed on several localities due to erosion, and its chemical and mineral composition is presented in this paper. It is built of K‡feldspar, plagioclase, myrmekite, metasomatic albite, biotite, muscovite and quartz, while apatite, magnetite, monazite, allanite and zircon are present as accessory minerals. According to its chemical and mineral composition and rock chemistry (trace and REE elements) the Polumir granite is leucocratic, sin‡collisional, with S‡type characteristics. It crystallized at temperature of about 650°C and under pressure of 2‡4 kbar. Results of isotope analyses (K‡Ar method on biotites) indicate that the Polumir granite was formed during the Miocene (14‡19 Ma) and it has undergone subsequent weak remobilization afterwards.


INTRODUCTION
The Polumir granite is located in the southeastern part of the ^emerno Mt. (1200 m above the sea level) between Pade` and the river Ibar (Fig. 1).Due to erosion, it has been exposed on several localities: Duboki potok, Lu~ki potok, Mr{ave livade, Venac and Pade`.Fine ‡grained granites, mainly of vein character, as well as pegmatites and aplites from the central part of the ^emerno Mt. (1300 m above the sea level), are not presented in this paper.Chemistry and petrology of the Polumir granite has been poorly studied and thus poorly known until now.Before sixties, these acid igneous rocks were only mentioned in passing, or they were studied from a general point of view (Maksimovi} & Divljan, 1953;Simi}, 1956;Simi}, 1957).The first petrological data were given by Uro{evi} et al. (1966) and Brkovi} et al. (1977).A petrological description of the Polumir granite was given by Vukov (1980, 1981, and later in 1995).Panto et al. (1988), discussed chemical composition and origin of accessory minerals.Nikoli} & Poharc ‡Logar (1984) investigated the NH 4 ‡ion content in biotittes from the granitic rocks at some of the mentioned localities.Results of petrological and chemical research on granites from majority of the mentioned localities are presented in this paper.According to the obtained data, physical ‡chemical conditions of crystallization of these granites, as well as their genetic ‡geotectonic setting, are analysed.

ANALYTICAL TECHNIQUES
Major and trace element compositions of the Polumir granite were determined by X ‡ray fluorescence spectrometry (XRF), while rare earth elements (REE) concentrations were measured by Inductive Coupled Plasma spectroscopy (ICP) at the Federal Institute for Geosciences and Natural resources in Hanover (Germany).Major and trace element compositions were determined in 5 granite samples (Table 1) from Duboki potok, Lukovski potok, Pade` and Mr{ave livade (Fig. 1).

GEOLOGY
The Polumir granite is a geological body which is, due to erosion, exposed et seven localities that cover the area of about 2.5 km 2 .It was considered to be a concordant intrusion in the "Mr{ave Livade Seriae" (Simi}, 1957), i.e. in the upper part of the Upper Paleozoic Lower Studenica Series (Simi}, 1956) which was primary composed of clayey, marly, sandy and carbonate sediments, with subordinated diabases and tuffs.Endomorphic changes within granites are weak.Chilled margins, thermal metamorphism and contact ‡metasomatism, as consequences of the temperature differences between granites and the surrounding rocks, were also noted.
Observing the contact ‡metamorphic changes and strongly sheared contacts between the Polumir granite and the surrounding rocks, variable degrees of K ‡feldspar latice ordering, plastic deformations of micas and feldspars, the Polumir granite was intruded into this level as a crystallized mass for the most part, or as reomorphic, heated and uplifted granitic mass (Vukov, 1981).

PETROGRAPHY
The Polumir granite is medium ‡grained (1 ‡3 mm) rock, light ‡gray to palepink in colour, sometimes darker (with clearer schistosity), with hypidiomorphic granular texture.It has a schistose structure, usually well ‡expressed, especially on margins, i.e. on the contacts with surrounding rocks.
K ‡feldspars occur as grains up to 3 mm in size, and they make about 28 to 42% vol. of rock.They contain perthite exsolutions.K ‡feldspars are sometimes transformed into microcline or mirmekite.Large K ‡feldspar grains (up to 2 ‡3 mmin size) often enclose plagioclase, biotite and muscovite, and are almost always cataclased or deformed.
Plagioclases make from 12 to 25% vol. of the Polumir granite.They occur as deformed and cataclased individual subhedral grains up to 5 mm in size (usually about 2 mm).They are associated either with orthoclase grains with often intergrown myrmekite, or they occur as individual grains in a fine ‡grained quartz mass.
Biotite is presented by banded, parallel oriented, lath ‡like grains with average size of about 1 ‡2 mm.It makes about 5 ‡8% vol. of rock.Biotite is sometimes slightly chloritized and limonitized with exsoluted dusty magnetite along the cleavage planes.
Muscovite is less abundant than biotite and makes from 0.5 to 4.5 % vol. of rock.Muscovite occurs as banded, lath ‡shaped flakes up to 2 mm in size, usually associated with biotite, whereas post ‡kinematic muscovite is rare.
Quartz makes from 34 to 45% vol. of rock.It is present, either as individual grains up to 2 mm in size, which show undulatory extinction, or as fine ‡grained aggregates between large feldspar or mica grains.Fine ‡grained, monomineral and lense shaped aggregates of quartz, up to few cm in length, were also observed.
Among the accessories, apatite, zircon and magnetite occasionaly occur as inclusions in biotite, and they are rarely associated with muscovite.They were also observed in rock mass, and their grain size never exceeds 0.3 mm.
Fine ‡grained sericite is the dominant secondary mineral which replaces mostly K ‡feldspar, while less abundant chlorite replaces biotite.

CHEMICAL COMPOSITION OF MINERALS
Chemical composition of minerals was determined on samples from the mentioned localities, which were selected after a detailed microscopic study.Special attention was payed to representative samples, chosen according to the content and size of main/principal minerals, fabric and alteration degree.As it has been already mentioned, chemical composition of K ‡feldspar, plagioclase, biotite and muscovite was determined.Explanations K ‡feldspar.Chemistry of this mineral was determined (Or 82.5 ‡92.9 Ab 7.1 ‡17.3An 0.0 ‡0.3 ) on fresh or slightly altered (replaced by sericite or clay minerals) grains, over 1 mm or smaller in size.Chemical composition of K ‡feldspar with plagioclase, biotite, muscovite and quartz inclusions was also determined, but it didn't show any regularity.
Because the K ‡feldspars are transformed to a different extent, either in microcline (from 0.10 ‡0.12 to 0.75 ‡0.93) or in perthite, they are of very variable composition.The largest amount of sodium, from 1.4 ‡1.93, was noted in the hypsometrically highest part of the granite body, i.e. at Mr{ave livade and at Pade` (Table 2, analyses 13 ‡17, Fig. 4).

CRYSTALLISATION CONDITIONS AND GENETIC ‡GEOTECTONIC SETTING OF THE POLUMIR GRANITE
Chemistry of biotite, muscovite and feldspar from the Polumir granite enabled determination of P ‡T conditions of the magma crystallization.

TiO
Fe O MgO Na:(Na+K) f r e q u e c y Biotite crystallized, according to its chemical composition, in stable P ‡T conditions, at relatively shallow depth and at moderate ‡to ‡low temperature (Fig. 7).High /f=Fe:(Fe+Mg)/ ratio, within the range 65 ‡73%, suggests a crystallization temperature for biotite of about 650°C (Ivanov, 1970).High content of NH 4 ‡ions (254 ppm) in biotite (Nikoli} & Poharc ‡Logar, 1984) is typical for the S ‡type granite.
Concerning the ratio between TiO 2 , Fe 2 O 3 and MgO, muscovite most likely crystallized directly from the melt (together with biotite), rather than by a post ‡magmatic rinsing (MgO etc.), although some muscovite from the upper part of the Polumir granite pluton (Mr{ave livade, Pade`) contain smaller amount of sodium (XNa=6.44‡6.52), which makes them close with post ‡magmatic muscovite (Monier et al., 1984, Fig. 6b).The existence of the primary muscovite is in accordance with the literature data (Anderson & Rowley, 1981;Monier et al., 1984).The eutectic minimum for the Polumir granite, calculated on the basis of normative QAbOr, was at T=670 ‡655°C under P=2 ‡4 kbar (Fig. 9).Similar or identical crystallization conditions are suggested for the crystallization of muscovite.The two ‡feldspars geothermometer of Stormer (1975) applied to the Polumir granite gave temperatures of about 400 to 550°C for perthite exsolution.Formation of perthite within K ‡feldspar (with Ab 16 ‡17 ) in the highest part of the Polumir granite (Pade`, Mr{ave livade) where the plagioclase is the most basic (with up to Ab 68 ), ceased at the temperature of about 550°C.More intense transformation of K ‡feldspar (Ab 7 ‡10%) associated with plagioclase (nearly 80% of Ab) into perthite occured in the lower parts of the pluton (Duboki and Lu~ki potok), at temperature of about 400°C.This was probably connected to subsequent metamorphism ‡ remobilization (together with shearing, microclinisation and, probably weak Si ‡Na ‡metasomatism).Calculated temperature using the supposed pressure of 2 kbar, corresponds to the eutectic minimum of P=2 ‡4 kbar (Fig. 9).According to the mineral and chemical composition, the Polumir granite originated by melting of continental crust and, according to various criteria, it has the S ‡character (Chappel & White, 1974;Ne~aeva, 1976;White & Chappell, 1977;Pearce & Gale, 1977;Takahasi et al., 1980;White et al., 1982White et al., , 1986;;Collins et al., 1982;Rub et al., 1983;Chappell, 1984;Bowden et al., 1984, etc.).

CONCLUSION
According to its mineral and chemical composition, the Polumir granite belongs to S ‡type granites.It crystallized from the magma which had been generated by partial melting in the upper levels of continental crust.During the Paleogene, in the Vardar Zone, i.e. in the collisional zone between two continental blocks, the Eastern microplate and the Drina ‡Ivanjica Element granitic rocks were intruded.Afterwards, under the extensional conditions during the Miocene, partial melting occurred and new magma was generated.These melts, analogous to S ‡type magmas, were intruded into higher level, and the Polumir granite originated by their crystallization.The granite was remobilized several times when the fine ‡grained granites and pegmatites from the central part of the ^emerno Mt. were probably formed.

Fig. 7 .
Fig. 7.I ‡f diagram for biotite composition from the Polumir granite as a function of chemical potential of K and water (I v a nov, 1970).f=Fe t /(Fe t +Mg); l=Al/(Si+Al t +Fe t +Mg).Sl. 7. I ‡f dijagram sastava biotita iz stena

Fig. 11 .
Fig. 11.The rare earth elements content within the Polumir granite normalised to average compostiion of continental crust (T a yl o r & M c Le n na n , 1985).

Table 4 .
Chemical composition of biotite.

Table 5
Chemical composition of muscovite.