Delimitation of a cryptic Eocene tectono-thermal domain in the Eastern Cordillera of the Bolivian Andes through K–Ar dating and 40 Ar– 39 Ar step-heating

Abstract

New K–Ar ages and 40 Ar– 39 Ar age spectra for samples from granitoid rocks and meta-sediments of the Cordillera Real, Bolivia, reveal a complex mid-Tertiary tectono-thermal history for this segment of the Central Andean eastern cordillera. Previous radiometric dating of the Cordillera Real intrusions defined two, areally-distinct, plutonic domains: in the N, Upper Triassic–Lower Jurassic; in the S, Upper Oligocene–Lower Miocene. Our new K–Ar ages, many from deformed intrusions, fall between these two episodes. For the variably foliated Zongo massif, there is a marked discordance between biotite and muscovite K–Ar dates, each decreasing consistently from SW to NE across this 8 km wide plutonic centre. The age gradients are developed within the foliated and unfoliated facies of the Zongo intrusions, but do not continue NE into the Palaeozoic meta-sedimentary rocks, for which K–Ar dates are erratic. 40 Ar– 39 Ar step-heating analyses of muscovites and most biotites from granitoid rocks reveal that the oldest and youngest micas yield plateaux (210 and 39 Ma), whereas those with intermediate ages exhibit disturbed spectra. A plateau date for a biotite from the NE margin of the Zongo massif shows that final cooling through c. 300 °C occurred in the Late Eocene. The new data reveal that a marked thermal discontinuity existed 39 Ma ago at the NE contact of the Zongo intrusion. Additional discordant dates for the Yani and Illampu intrusions, to the NW, indicate that the discontinuity was of regional scale. There is no direct relationship between this tectono-thermal event, which affected at least 2000 km 2 , and the Upper Palaeozoic foliation of several of the plutons involved, and thus it is cryptic on outcrop and microscopic scales. We propose that the thermal discontinuity resulted from Late Eocene crustal shortening, SW-verging thrust faulting, and attendant cordilleran uplift at the inner (eastern) boundary of the Andean orogen, coincident with the Incaic deformation defined in Peru.