engleski

Ar/Ar dating of muscovite and fluid inclusion study of quartz from the pegmatite of Srednja Rijeka, within granitoids of Moslavačka gora Mt., North Croatia

The Mt.Moslavačka gora crystalline complex is located in the southwestern part of the Tisia unit, representing a geotectonic fragment of the Carpathians. The complex is composed of granite pluton (100 km2) which is conformably floored and roofed by migmatites and bordered by an Abukuma-type metamorphic sequence, indicating the synkinematic character of the granite plutonism (Pamić, 1990). Pegmatites are very common within the Mt. Moslavačka gora granite pluton, indicating their mutual genetic relationship. Major minerals of the pegmatites are microperthite, microcline, orthoclase, albite, quartz, biotite, muscovite, and in some places tourmaline (Tučan, 1904). As a rule, the pegmatites occur within the granitoid pluton, which is composed of S-type varieties, usually in the form of dcm-cm sized lenses and irregular bodies, frequently with diffuse contact with the urrounding granites, indicating their secretional nature (Pamić, 1990). The pegmatites from Srednja Rijeka, on the northern flanks of Mt. Moslavačka gora, close to the Čazma town, however, are well developed veins of micropertite, microcline, orthoclase, albite, quartz, tourmaline, biotite and muscovite, surrounded by an aplitic haloes with the same mineral composition, and medium grained, two-mica, tourmaline rich granites. Pegmatite quartz containes numerous fluid inclusions (FIs). So far three type of FIs have been recognized, high density-low salinity and high density-high salinity aqueous-carbonic FIs and aqueous FIs. In the former degree of fill (water) varies between 0.45 and 0.60. TfmCO2 occurs in the temperature range between -74.0 and -76.0 oC and Tm CO2 is observed between -56.9 and -65.5 oC, what is assigned to the presence of other volatiles (CH4, N2). TmIce is in the interval between -2.0 and -9.0 oC . Tm Clathrate is between 6.5 and 7.0 oC, what corresponds to 5.8 and 6.2 wt.% equ.NaCl, assuming negligible clathrates of N2 and CH4 has been formed. Homogenization of the carbonic phase proceeds into liquid phase in the temperature range between 19.2 and 20.0 oC. The temperature of total homogenization into liquid phase, ThTOT, occurs between 340 and 360 oC. FIs study is still in progress. The first age determination was done by Deleon (1969), who obtained two Rb/Sr model ages of 92 and 62 Ma on migmatites. Afterwards, K/Ar dating , carried out on biotites and hornblendes from granitoids, migmatites, and amphiboles from the Abukuma-type metamorphic sequence yielded ages ranging between 88 and 61 Ma, whereas K/Ar ages of 75 and 57 Ma were produced on biotites from the pegmatites (Pamić, 1990). All these data indicate Alpine age of the Mt. Moslavačka gora crystalline rocks. However, 87Sr/86Sr and 87Rb/86Sr ratios were measured on the fresh whole rock samples subsequently, taken from the granite pluton, but a great scatter of data did not allowed construction of the appropriate isochron nor age calculation. The measured 87Sr/86Sr was very high, up to 0.74302, indicationg pre-Alpine, probably Variscan cycle surely, and then later the pluton was pervasively deformed and overprinted during the Alpine orogeny (Late Cretaceous), as indicated by the lower K/Ar ages (Lanphere and Pamić, 1992). Ar/Ar dating: the muscovite from Srednja Rijeka pegmatite was irradiated in the Budapest research reactor, tank type, of the Atomic Energy research Institute. Thermal neutrons were absorbed by a Cd shield of 0.5 mm thickness, the irradiation parameter was J=1.172x10-3. LP-6 biotite (Age: 128.5 Ma, Roddick, 1983) was used as a standard. Special features of experimental techniques, used in this laboratory were described by Balogh and Simonits (1968), gas purification and mass spectrometric measurements by Balogh (1985). The results on interlaboratory standards GL-O can be found in Odin et al. (1982). The muscovite from Srednja Rijeka pegmatite gave a plateau age of 73.2 +0.8 Ma. No overprint or excess Ar is indicated by the age spectrum and thus the plateau age can be considered as the time of muscovite crystallization and/or cooling. This Ar/Ar plateau age of the pegmatitic muscovite fits with Alpine K/Ar ages obtained on the granitoid country rocks and the surrounding migmatites and the. amphibolite facies metamorphic rocks. Based on the presented data one may conclude that crystallization of the pegmatite muscovite and the generation of the Mt. Moslavačka gora pegmatites themselves, were related to post-collisional evolution of the granitoid pluton. It crystallized during the main Variscan collisional event somewhere along the southern margin of Laurussia. In the Middle/Late Jurassic, the Tisia fragment break off from the South Laurussia and was transported southwardly to its present structural position (Pamić, 1998). Its subsequent geodynamic evolution was related to interaction with the northern margin of the Dinaridic Tethys, i.e., during subduction of the Cretaceous/Early Paleogene oceanic crust of the Dinaridic Tethys beneath the South Laurussia and Tisia. The long-lasting subduction processes (25-30 Ma), fundamentally changed geothermal regime of the overlying Tisia. It gave rise to "rejuvenation" of Mt. Moslavačka gora granite body, brought about loss of radiogenic Ar, and caused generation of pegmatites. Hence, the formation of the pegmatites within the Paleozoic Moslavačka gora granite body was penecontemporanous with the processes of Alpine overprinting.

Ar/Ar dating; pegmatites; fluid inclusions;Mt. Moslavačka gora; North Croatia

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