Resumen de Low temperature thermochronology of apatites with exceptional compositional variations: the Stillwater Complex, Montana revisited

Barry P. Kohn, David A. Foster, K.A. Farley

• The halogen content of apatite (principally F and Cl) has a strong effect on the annealing properties of fission tracks. Building on an earlier study we further examine the thermochronological consequences of this behaviour in the Archaean Stillwater Complex, Montana, a layered mafic intrusion which displays (sometimes at the handspecimen level) a wide range of apatite Cl composition (-0- 7.6 wt% Cl). Further detailed sampling confirms our previous findings of a positive correlation between apatite fission track (AFT) ages (-63 Ma to >500 Ma) and Cl wt% (-0.01 to 6 wt % Cl). The most Cl-rich apatites (-6-7.6 wt% Cl) however, generally yield AFT ages similar to fluorine-rich apatites (late Cretaceous-early Tertiary). These AFT ages are considered to date regional cooling during the Laramide Orogeny. This shift is tentatively attributed to a change in annealing behaviour accompanying a transition from hexagonal to monoclinic symmetry in the most Cl-rich apatites. Apatite single grain (U- ThflHe data from samples containing a range o f Cl compositions all yield late Cretaceous-early Tertiary ages. This age range is concordant with the AFT data fo r both the most fluorinerich and chlorine rich apatites, and suggests that apatite composition does not affect the sensitivity of the He partial retention zone. K-feldspar 40Ar/3vAr thermochronology from an Archaean granitic intrusion in the complex suggests that maximum Phanerozoic palaeotemperatures for the Stillwater never exceeded-140°- 150°C.