Blue amphibole, Arthur Metamorphic Complex, Tasmania: Composition and regional tectonic setting

Abstract

Blue amphibole minerals, including glaucophane, were formed in the Arthur Metamorphic Complex of northwest Tasmania during the early part (514–510 Ma) of the Tyennan Orogeny (514–490 Ma ∼ Delamerian Orogeny). These blue amphiboles are now a rare relict phase in the predominantly greenschist facies Bowry Formation. The hosts range from an essentially prograde metadolerite to a substantially retrograde, mafic schist. Chemical parameters of the glaucophane indicate that it formed under minimum conditions of 350°C and 700 MPa, reflecting burial in a subducted wedge. Extrusion of the Bowry Formation from the wedge, with associated re‐equilibration and deformation, commenced at 510 Ma (Early Cambrian). The formation was initially emplaced at a high crustal level, where it may have formed part of a ductile zone at the base of westward moving, upper crustal nappe of relatively unmetamorphosed Neoproterozoic to Lower Cambrian sedimentary, basaltic and ultramafic rocks. The Bowry Formation now occurs in the eastern boundary zone (Arthur Metamorphic Complex) of the Rocky Cape cratonic fragment. Other high‐pressure metamorphic rocks of approximately the same age, but of deeper crustal origin, were emplaced at middle to upper crustal levels in the Tyennan and Forth regions, to the east of the Arthur Metamorphic Complex. These rocks were exposed by middle Middle Cambrian times (506 Ma) in domes that, along with the Rocky Cape craton, controlled the arcuate form of Middle to Late Cambrian sedimentary troughs.