GROWTH AND STABILIZATION OF CONTINENTAL CRUST BY BACKARC OPENING AND CLOSING IN THE PALEOZOIC TASMANIDES OF EASTERN AUSTRALIA

Petrological/geochemical tracking of geodynamic process throughout the evolution of the vast Paleozoic Tasmanide orogenic system (TOS) of eastern Australia indicates that it grew and stabilized during a series of alternating extension/contraction events associated with backarc opening and closing, not during terrane accretion.

The TOS is a series of outboard younging, turbidite-granite orogens comprising the inboard Delamerian (530–500 Ma), medial Lachlan (490–350 Ma), and outboard New England (350–240 Ma) orogens. Remnants of an outboard-migrating, leading-edge arc sequence are recognizable throughout the history. Long-term extensional events produced a series of extensive turbidite-filled, backarc basin sequences, separated by the arc remnants. Intermittent backarc closure generated short-lived (~10 Ma), narrow fold-thrust belts, intruded by syn- and post-kinematic granites.

Four major extensional episodes are recognized as a repeated tripartite lithological association, each of which initiated the major orogens. These associations generally consist of (i) inboard S-type granite, (ii) outboard oceanic arc, and (iii) intervening, turbidite-filled backarc basin. S-type granites herald the formation of a new, outboard oceanic arc and extensional backarc, following closure of the old backarc basin during flat subduction. S-type plutonism was triggered by mafic magma injection during renewed arc magmatism, once slab retreat was re-established. I- and A-type granites respectively follow during ongoing slab retreat, as the crust progressively thins and the backarcs become distal.

The petrological evolution is mimicked through basalt compositions and isotopic evolution. The arc-backarc transition with S-, I-, to A-type granite is also evident as decreasing LILE/HFSE ratios that indicate a progressive loss of slab flux character in the basalts. WR Nd and Hf isotopic ratios in granite-hosted zircon also show a gradual change from crustal to depleted mantle character with progressive slab retreat. The evolutionary isotopic pattern is repeated with each contraction-extension cycle. These distinctive geochemical/petrological cycles are the hallmark features of backarc opening and closing in retreating accretionary orogens.