A new contribution to the geochronology of Mt. Moslavačka Gora (Croatia)

The crystalline complex of Mt. Moslavacka Gora is situated in the area between Sava and Drava rivers in the south part of the Pannonian Basin and belongs to the South Tisia. It consists of a S-type granitic pluton (~110 km2) surrounded by migmatite and medium grade metamorphic rocks which also form enclaves up to more than hundred meters in size (PAMIC 1990). The magmatic rocks comprise monzogranites, andalusite- and sillimanite-bearing granites, granodiorites, and andalusite-tourmaline-bearing pegmatites. Also occurs ambiguous olivine-gabbro of unknown primary position being affected by high temperature-low pressure metamorphic overprint. Based on geothermobarometry and phase relations the metamorphic rocks exhibit two main metamorphic imprints (GARASIC 1993, BALEN 1999). The older medium pressure metamorphism (T=550-820°C, P=5.2-9.6 kbar) reached values up to amphibolite-granulite transition. It produced sillimanite-biotite-garnet paragneisses, mica schists, amphibolites and marbles that formed contemporaneously to the main structural imprint of the rocks. A high temperature-low pressure metamorphic overprint is related to the pervasive recrystallization of migmatites and S-type pluton. Metamorphic conditions of 550-650°C at 2.5-4.0 kbar are indicated by the occurrence of clinopyroxene and grossulare-rich garnet in amphibolites, clinohumite replacing forsterite in marbles and cordierite replacing biotite, sillimanite and almandine in cordierite-bearing gneisses. DELEON (1969) made the first radiometric data: two Rb-Sr model ages on granites (90 ą 5 Ma, 62 Ma) and one on gneiss-granite (64 Ma) emphasizing that this data do not indicate primary crystallization ages but rejuvenation. LANPHERE & PAMIC (1992) obtained Alpine K-Ar ages on biotite from oil-well granitoid rocks (72.7ą2.2 to 27.0ą2 Ma), and also on biotite from outcropping migmatite (70.5ą2.2), granodiorite (70.0ą2.0), pegmatite (57.0ą2.0), and on hornblende from amphibolite (88.0ą3.0 Ma). On the basis of measured very high 87Sr/86Sr ratio (0.74302ą5) authors concluded that the granite originated in pre-Alpine times and underwent pervasive deformation and overprinting during the Alpine orogeny. PALINKAS et al. (2000) obtained Ar-Ar age on muscovite from Srednja Rijeka pegmatite with plateau age of 73. 2ą0. 8 Ma and considered it as the time of muscovite crystallization and/or cooling. This paper presents additional geochronological age data. Sm-Nd isochrons were measured from an olivine gabbro (Kamenjaca) and a garnet-andalusite bearing pegmatite. The olivine gabbro is 109ą8 Ma in age and the pegmatite (Srednja Rijeka) yielded a badly defined Sm-Nd isochron of 77ą33 Ma. Plateau type Ar-Ar cooling ages on hornblende of the amphibolites (Pleterac and Zorovac) are 83ą2 Ma and 81ą2 Ma respectively, and those of muscovite obtained from the pegmatite is 74ą1. 0 Ma (Fig. 1). At present the age of the first imprint of the metamorphic rocks from Mt. Moslavacka Gora is still unknown. Based on petrological-geological similarity with Slavonian Mts. (PAMIC 1998) the Variscan age is likely, but an early Eoalpine age can not be excluded. Geochronological data point to the Cretaceous age of the S-type pluton and the related metamorphic event. According to the presented data the granitic intrusion has to be older than the Ar-Ar cooling ages (~ 80 Ma). Cooling below 400°C occurred at about 75 Ma as indicated by the Ar-Ar muscovite ages from pegmatite. Olivine gabbro is the oldest known rock and its role in evolution of studied area is not clear. Eventually it may present an allochthonous part of contemporaneous ophiolites from the nearby Sava-Vardar Zone of Dinarides.