MINERALOGY AND TEMPORAL RELATIONS OF COEXISTING AUTHIGENIC MINERALS IN ALTERED SILICIC TUFFS AND THEIR UTILITY AS POTENTIAL LOW-TEMPERATUREDATEABLE MINERALS

Coexisting fine-grained (0.1-20 mu m) authigenic silicate minerals sep arated from altered tuffs in Miocene and PlioPleistocene lacustrine de posits were characterized petrographically and using X-ray powder diff raction. The authigenic minerals are dominated by clinoptilolite, erio nite, phillipsite, K-feldspar, silica, calcite, smectite, and randomly interstratified illite/smectite. Minor accessories of opal-CT, cristo balite, and barite are present with the major alteration minerals. Aut higenic minerals from altered tuffs were dated using the K/Ar method t o evaluate the utility of these minerals for determining the time of a lteration in low-temperature diagenetic environments. The eruption age s of some of these zeolite-rich tuffs were determined using the Ar-40/ Ar-39 method on single sanidine and plagioclase minerals. The K/Ar iso topic ages of the fine-grained K-feldspar show minimal variation compa red with results from the clinoptilolite separates. The isotopic ages from the authigenic K-feldspar (15-13.8 Ma) and some of the zeolites ( 16.2-6.7 Ma) are similar to the eruption ages of the tuffs and indicat e early alteration. Despite their open-framework structure, zeolites a pparently can retain part or all of their radiogenic argon under favor able conditions (e.g., saturated environment). How much of the radioge nic argon is retained is estimated from the isotopic ages of other coe xisting secondary minerals that are commonly dated by the K/Ar method. Although zeolite isotopic ages should be interpreted with caution, th ey may be useful to constrain temporal relations of low-temperature di agenetic processes when used in conjunction with other dateable minera ls.