K-Ar age constraints on the sources of K minerals in bentonites of the Ankara-Cankırı Basin, Central Anatolia, Turkey

摘要

Many of the bentonite deposits of the Ankara-cankiri Basin, Central Anatolia, Turkey were found within the Miocene Hancili Formation, which comprises lacustrine sedimentary and volcaniclastic rocks that interfinger with Uludere pyroclastic rocks of the Miocene Galatean volcanic province. In the present study, the conventional K-Ar geochronological method was used to evaluate the contribution of volcanic materials to the K-bearing components of the bentonite clay fractions. Four dacite samples from near the southern end of the basin were indistinguishable in K-Ar age (average 18.4 Ma, standard deviation 0.3 Ma). K-Ar measurements of feldspar-enriched rock fragments and hydrobiotite separated from andesitic tuff from near the northern end of the basin indicated an age of 17 +/- 1 Ma. The K-Ar age values of clay fractions of bentonites, which ranged from 77 +/- 5 Ma to 162 +/- 5 Ma, indicate that most of the K in the bentonite clay fractions occurs in minerals derived from the Mesozoic basement rocks adjacent to the Miocene basin. The K-Ar age values support field observations indicating that these bentonites are secondary bentonites formed by alteration of volcanic components during or after deposition of volcaniclastic phases. The K-bearing mineral components of these clay fractions consisted mostly of unaltered illitic material of detrital origin whereas the smectitic components were formed by alteration of Miocene pyroclastic materials.