DATING PALEOHYDROLOGIC EVENTS WITH AUTHIGENIC APOPHYLLITE: AN EXAMPLE FROM THE PERMIAN OF THE CENTRAL TRANSANTARCTIC MOUNTAINS, ANTARCTICA

Determining the ages of post-depositional events in sedimentary strata is often hampered by the lack of suitable materials for analysis. Apophyllite, a K-bearing hydrous sheet silicate, is a well known low temperature (≤100°C) secondary mineral associated with zeolites in vugs in basalt lavas, and is reported to occur in volcaniclastic strata. Sandstones of the Permian Buckley Formation contain plagioclase and volcanic rock fragments; laumontite occurs widely (in the matrix and replacing plagioclase) and has been attributed to zeolite grade metamorphism related to emplacement of the Jurassic (180 Ma) Ferrar Dolerite sills. Secondary mineralization, developed in a Buckley channel sandstone at Rougier Hill, includes quartz, prehnite and apophyllite. The apophyllite occurs as masses of bladed crystals which envelope garnet and vesuvianite that must record an earlier metamorphic event, probably associated with the Jurassic sills. The Buckley Formation apophyllite gives ca. 65 Ma 40Ar/39Ar dates that are significantly younger. Burial depth at the time of formation was probably 2.0-3.0 km depending on the extent of pre-Cenozoic uplift and erosion. Apophyllite also occurs in Jurassic Kirkpatrick Basalt lavas and yields concordant K-Ar and Rb-Sr mid-Cretaceous ages (Fleming et al., 1999) which are interpreted to reflect the time of mineral formation and to have been related to the early stages of uplift of the Transantarctic Mountains and the associated changes in the hydrologic regime. The 65 Ma age for the Buckley Formation apophyllite is interpreted similarly to reflect younger mineralization events consequent on uplift. Fission track ages (Fitzgerald, 1994) indicate episodic uplift of the region beginning in the Cretaceous (ca. 115 Ma) and continuing through the Cenozoic. Although issues remain with apophyllite dating, it is a mineral that may be useful for dating low temperature paleohydrologic events in sedimentary sequences.