40Ar/39Ar laser probe dating of detrital white micas from Cretaceous sedimentary rocks of the Eastern Alps: Evidence for Variscan high-pressure metamorphism and implications for Alpine orogeny: Comment and Reply

Abstract

The detritus of Cretaceous synorogenic sandstones of the northern margin of the Austroalpine microplate contains evidence for a high-pressure metamorphic basement and obducted oceanic crust exposed in early Alpine time. 40 Ar/ 39 Ar laser-probe data of detrital white micas give excellent plateau ages in a narrow range from 320 to 360 Ma. White micas cover the whole range from muscovites up to phengites (3.04 to 3.48 Si per formula unit). Heavy mineral spectra contain chrome spinel, glaucophane, chloritoid, epidote, and garnet, as well as zircon, tourmaline, and rutile. Glaucophane, chloritoid, and phengite correlate in their abundance. These minerals also correlate positively with the stable minerals zircon, tourmaline, and rutile derived from continental rock and negatively with chrome spinel, which represents oceanic crustal provenance. We propose that both glaucophane and phengite come from the same source rock, Variscan high-pressure metamorphic rocks of the Austroalpine basement. The Early Carboniferous age gives constraints for paleogeographic models concerning the collision between Gondwana and Laurussia. Furthermore, detrital glaucophane in the early Alpine sedimentary rocks cannot be used as proof for Cretaceous subduction at the Austroalpine-Penninic plate boundary.