LATE PRECAMBRIAN TECTONOTHERMAL EVOLUTION OF THE MALVERNS COMPLEX

The Malverns Complex is a variably deformed and metamorphosed late Pre cambrian calc-alkaline plutonic suite exposed within the Avalon terran e of central England. Published U-Pb zircon and monazite ages indicate emplacement at c.680-670 Ma. Ar-40/Ar-39 mineral cooling ages present ed here place constraints on the timing of: (1) upper greenschist to l ow amphibolite facies metamorphism and associated ductile deformation; and (2) late hydrothermal activity and variable thermal reactivation. Hornblende from meta-diorites records Ar-36/Ar-40 v. Ar-39/Ar-40 isot ope correlation ages of c. 649 Ma and c. 652 Ma. These are significant ly younger than the crystallization age of the complex and are interpr eted to date cooling following metamorphism. They further provide maxi mum age constraints for the development of mylonitic fabrics that over print metamorphic textures. Hornblende from a diorite net-veined by la te granite pegmatites that are discordant to the mylonitic fabrics rec ords a significantly younger isotope correlation age of c. 610 Ma. Thi s is interpreted to date a static thermal rejuvenation associated with pegmatite emplacement, and places minimum age constraints on the timi ng of mylonitization. A plateau age of c. 597 Ma recorded by muscovite from a greisen vein within a granite is interpreted to date closely t he hydrothermal alteration of the complex. Detrital muscovite of proba ble metamorphic origin within a Lower Cambrian sandstone records a pla teau age of c. 598 Ma, and may have been derived from local units whic h were first metamorphosed at c. 650 Ma and later reheated at c. 610-6 00 Ma. The Ar-40/Ar-39 ages are consistent with episodic tectonotherma l activity within the Malverns Complex during the interval c. 650-600 Ma, and indicate a more complex Precambrian history than previously re cognized for the Avalon terrane of southern Britain.