AR-40 AR-39 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/

The detritus of Cretaceous synorogenic sandstones of the northern marg in of the Austroalpine microplate contains evidence for a high-pressur e metamorphic basement and obducted oceanic crust exposed in early Alp ine time. Ar-40/Ar-39 laser-probe data of detrital white micas give ex cellent 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 S i per formula unit), Heavy mineral spectra contain chrome spinel, glau cophane, chloritoid, epidote, and garnet, as well as zircon, tourmalin e, and rutile. Glaucophane, chloritoid, and phengite correlate in thei r 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 pr ovenance. We propose that both glaucophane and phengite come from the same source rock, Variscan high-pressure metamorphic rocks of the Aust roalpine basement. The Early Carboniferous age gives constraints for p aleogeographic models concerning the collision between Gondwana and La urussia. Furthermore, detrital glaucophane in the early Alpine sedimen tary rocks cannot be used as proof for Cretaceous subduction at the Au stroalpine-Penninic plate boundary.