Late Precambrian tectonothermal evolution of the Malverns Complex

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Abstract
The Malverns Complex is a variably deformed and metamorphosed late Precambrian calc-alkaline plutonic suite exposed within the Avalon terrane of central England. Published U–Pb zircon and monazite ages indicate emplacement at c. 680–670 Ma. 40 Ar/ 39 Ar mineral cooling ages presented here place constraints on the timing of (1) upper greenschist to low amphibolite facies metamorphism and associated ductile deformation; and (2) late hydrothermal activity and variable thermal reactivation. Hornblende from meta-diorites records 36 Ar/ 40 Ar v. 39 Ar/ 40 Ar isotope correlation ages of c. 649 Ma and c. 652 Ma. These are significantly younger than the crystallization age of the complex and are interpreted to date cooling following metamorphism. They further provide maximum age constraints for the development of mylonitic fabrics that overprint metamorphic textures. Hornblende from a diorite net-veined by late granite pegmatites that are discordant to the mylonitic fabrics records a significantly younger isotope correlation age of c. 610 Ma. This is interpreted to date a static thermal rejuvenation associated with pegmatite emplacement, and places minimum age constraints on the timing of mylonitization. A plateau age of c. 597 Ma recorded by muscovite from a greisen vein within a granite is interpreted to date closely the hydrothermal alteration of the complex. Detrital muscovite of probable metamorphic origin within a Lower Cambrian sandstone records a plateau age of c. 598 Ma. and may have been derived from local units which were first metamorphosed at c. 650 Ma and later reheated at c. 610–600 Ma. The 40 Ar/ 39 Ar ages are consistent with episodic tectonothermal activity within the Malverns Complex during the interval c. 650–600 Ma, and indicate a more complex Precambrian history than previously recognized for the Avalon terrane of southern Britain.