Interpretation of 40Ar/ 39Ar and K/Ar dating evidence from the Aileu Formation, East Timor, Indonesia

Abstract

The Aileu Formation, East Timor, is a zoned metamorphic complex exposed in a modern arc—continent collision zone. During collision, extensive retrogression occurred. K/Ar and 40Ar/ 39Ar age measurements on hornblendes are interpreted as indicating that the retrograde metamorphism, and hence collision, occurred at ∼ 8 Ma ago, Late Miocene. Mica K/Ar ages from two samples reflect cooling of the terrain to < 300°C by 5.5 Ma ago. Relict Mg-rich hornblendes lost most of their pre-existing radiogenic Ar, and Fe-rich hornblendes were completely reset, during the retrogressive phase at middle amphibolite facies conditions. At lower grades of retrogressive metamorphism, the Mg-rich hornblendes retained appreciable radiogenic Ar, whereas Fe-rich hornblendes lost most of their accumulated radiogenic Ar even at upper greenschist facies. Thus, under these conditions, chemical composition is an important factor controlling Ar loss from hornblende, although recrystallization also plays a significant role.