Apophyllite Ar-40/Ar-39 and Rb-Sr geochronology: Potential utility and application to the timing of secondary mineralization of the Kirkpatrick Basalt, Antarctica

Apophyllite (KCa4Si8O20(F, OH). H2O), which formed as a low-temperature sec ondary mineral in the Kirkpatrick Basalt, has been examined to evaluate its potential for geochronology using the K-Br (by the Ar-40/Ar-39 technique) and Rb-Sr methods. The Ar-40/Ar-39 total-gas ages for apophyllites from the Kirkpatrick Basalt are 114-133 Ma for the central Transantarctic Mountains , 95-114 Ma for south Victoria Land, and 76-100 Ma for north Victoria Land. Within individual hand samples, apophyllite Ar-40/Ar-39 apparent ages show variations of up to 24 m.y., and within single large (up to similar to 2 c m) crystals, differences of up to 14 m.y. are observed. The Ar-40/Ar-39 inc remental-heating spectra are generally flat but have low- and high-temperat ure discordances attributed to Ar-39 recoil. Rb-Sr model ages for the apoph yllites range from 94 to 144 Ma and vary from concordant with the Ar-40/Ar- 39 dates to as much as 14 m.y, older. The dates are consistent with other t emporal indicators of low-temperature alteration in the basalts and are int erpreted to broadly reflect the time of apophyllite precipitation. They doc ument a geographically widespread early to middle Cretaceous secondary mine ralization which is inferred to be caused by a major period of groundwater movement associated with the initiation of uplift and denudation of the Tra nsantarctic Mountains. The results suggest that apophyllite can produce geo logically meaningful ages by both Rb-Sr and K-Ar methods, although further work is required to fully understand the variations in age that may be attr ibuted to Ar loss, alkali mobility, or an extended history of mineral preci pitation.