Spatio–temporal evolution of Mesoproterozoic magmatism in NE Australia: A hybrid tectonic model for final Nuna assembly

Abstract

The tectonic regimes that drove the 1560–1490 Ma granitic magmatism c. 50 m.yr. after the final assembly of the Proterozoic supercontinent Nuna in NE Australia remain elusive. Collision between NE Australia (Mount Isa Inlier—MTI) and NW Laurentia (Georgetown Inlier—GTI) occurred at c. 1600 Ma and was associated with a west-dipping subduction zone, with the MTI as the upper plate and the GTI as the lower plate. Structural studies in the GTI showed that the collisional event involved 1600 Ma WNW-ESE shortening, followed by 1550 Ma WNW-ESE directed extension. During this later stage, a crustal-scale, west-dipping detachment fault-system juxtaposed middle- to lower-crustal levels, associated with voluminous, 1550 Ma S-type granites against greenschist facies upper crustal rocks. Regionally, post-collisional magmatism defines a westward, chemical, and temporal trend from 1560 to 1550 Ma, dominantly S-type confined to the lower plate (GTI) through c. 1545–1540 Ma I-/A-type (below the Carpenteria Basin) to 1540–1490 Ma A-type granites that intruded further west the Australian upper plate (eastern MTI). This transition from hydrous (S-type) granites in the east to drier (A-type) granites in the west is also supported by increasing zircon saturation temperatures and geochemical discriminators. Recent zircon Lu–Hf and new in-situ monazite Sm–Nd analyses in granites show increasingly radiogenic initial (at the time of crystallization) isotopic ratios from the GTI to the MTI, reflecting a concomitant westward increase in mantle input. Combined, these features suggest a spatio–temporal evolution of hotter and drier crustal conditions westward associated with progressive lithospheric extension. Classical Phanerozoic upper-plate delamination, slab break-off, and slab rollback and/or tearing tectonic models do not account for all the features of this post-collisional magmatic record. Alternatively, a hybrid tectonic scenario between fast–hard Indian and slow–soft Aegean collision better explains the attributes of Mesoproterozoic NE Australia during Nuna assembly.