Abstract: In the Avnik area of the Bitlis Massif, the oldest rocks of the Lower Unit are a series of basic to felsic calc - alkaline metavolcanic rocks (454 ± 13 Ma), inter bedded with magnetite - apatite ores. These rocks are intruded by the Avnik ,and Yayla granitoids (347 ± 52 Ma) ,and unconformably overlain by an Upper Unit of micaschists ,and Permian marbles. The entire sequence was most recently deformed ,and metamorphosed during the Alpine orogeny. The metavolcanics ,and granitoids are extensively feldspathizcd ,and silicified.Although many of the metavolcanics retain porphyritic, spherulitic / volcaniclastic textures, their K`Na ratioshave been severely altered, to produce albitites ,and K-feldspar rocks. The probable cause of the feldspathization,which also affects the Avnik granitoids ,and the micaschists, is reaction between the rocks and the sea water that pervaded the basement during deposition of the overlying marine sediments. This metasomatic event is dated by a Rb - Srwhole-rock age (91 ± 9 Ma) on the metavolcanics, suggesting that the reactions coincided with the expulsion on theheated fluid phase during the Eoalpine orogenic phase- The rock/water reactions have altered the Rb/Sr ,and K/Rbratios, ,and homogenized Sr isotopes, but REE patterns appear to be undisturbed.REE patterns, combined with major - element chemistry, suggest that there is no genetic relation between thegranitoids ,and the metavolcanics. Metavolcanics may have been derived either by extreme fractional crystallizationof more basic, more plagioclase - rich magmas, / from anatexis of a feldspathic rock where plagioclase was an important part of the anatectic residue. A crustal anatectic origin is suggested for the granitoids, which is consistent withthe available Sr isotope data.