40Ar/39Ar Geochronology of Tephra and Volcanic Clasts in

AI)stract- 4nAr/39Ar analyscs of tepllra aiid clasts of volcanic rock proviclc age constraints for ~ ~ p p c r parts of the CRP-2A core. Single-crystal laser-fusion aiialyscs of ~~ ~ io r t l i~~c la se phenocrysts fro111 three tephra-bearing layers yielded the most precise age const~-z~ints for CRP-2A. The dated tcphra layers are: l) a 2.7-1x1-tliick interval of p ~ ~ m i c e and ash layers bet\veen l l 1.S and 114.2 meters belon ~ sea floor (mbso (weighted mean age = 21.44 2 0.05 Ma. 2 20): 2) a concentration of pumice near 193.4 inbsf (23.98 k 0.13 Ma): and 3) a con cent ratio^ ^ of p~ii~lice near 280 inbsf (24,22 5 0.03 Ma) (all ages are calibrated relative to Fish Canyon Tuff sanidine at 27.84 Ma), The l1 l to l l 4 inbsf tep111-a is alil~ost entirely non-reworked. and the 193 111bsf and 280 mbsf tephra concentratioils are interpreted as b e i i l ~ reworked and redeposited soon aflcr cruptioi~. All three of the tephra ages are therefore considerecl to be equivalent to depositional ages. The variation in precision of these three age deternlinations IS largely a function of phenocryst slze and ab~111cfance. The accuracy of these ages is equal to the accuracy of the cul-rent calibration of the 4oAr/:gA~- n~ethod (about * l %). +'nAr/39Ar results from volcanic clasts provide three additio~ial 11laxi111~im age constraints for the CRP-2A core. Single-crystal laser-fusion of sanidiiie pheiiocrysts from a rhyolitic clast fro111 294 nibsf yielded a precise ~naxim~im depositioi~al age of 24.98 & 0.08 Ma, and plateau ages of grou~ldinass concentrates froin basaltic clasts near 36.02 n~bs