Rb-Sr geochronology of the Bitlis Massif, Avnik (Bingöl) area, S.E. Turkey

In the Avnik area, felsic metavolcanics are interbedded with banded and massive apatite-rich iron ores, and are intruded by the Avnik and Yayla granitoids. These rocks are unconformably overlain with depositional contact by micaschists and (?) Permian marbles, which were folded and metamorphosed during the Alpine orogeny. The metavolcanics and the granitoids are extensively feldspathized and silicified. The Yala granite gives a poorly defined Rb-Sr age of 347 ± 52 Ma (IR = 0.7217 ± 80) but the albitized Avnik body gives only a scatter of data suggesting an age of 250-425 Ma. Feldspathized metavolcanics from a 100-metre section define an age of 91 ± 9 Ma. One sample of the Avnik granitoid gives an amphibole-rock-feldspar age of 71 ± 28 Ma and another gives a biotite-rock age of 41 ± 1 Ma; a micaschist gives a chlorite-muscovite age of 38 ± 2 Ma. An isochron age of 454 ± 13 Ma (IR = 0.7105) for metavolcanics in the Cacas area can be extracted from the data of Yilmaz et al. (1981). We interpret this as the age of eruption, and suggest a similar age for the Avnik metavolcanics. Our 90 Ma (Eoalpine) data on these metavolcanics requires metamorphic resetting of whole-rock Rb-Sr systems over tens of metres, and probably records the time of pervasive feldspathization and silicification. The data for the Yala granite are interpreted as the age of intrusion; the high IR indicates an origin by anatexis of older crust. The late-Alpine mineral ages probably reflect recrystallization during folding and thrusting.