The New England Fold Belt (NEFB) extends for ~ 2000 km along the eastern margin of Australia and is a tectonic collage of early to mid-Palaeozoic subduction/accretion elements and related late Palaeozoic extensional basins and structures, which are overprinted by the Permo-Triassic Hunter-Bowen Orogeny. Within the Marlborough terrane of the northern NEFB, thrusting associated with this latter event has exhumed an ultramafic-mafic complex, the largest (~700 km2 ) complex of this type known in eastern Australia.
Petrological, geochemical and geochronological studies reported in this thesis reveal that this ultramafic-mafic complex comprises products of 4 tectonomagmatic events. The first event is represented by elements of an ophiolite. These elements include both mantle peridotite (mostly serpentinised harzburgite plus minor serpentinised dunite-chromite pods) and crustal lithologies (mostly metamorphosed gabbros and diabases). Chromium spinel from both the residual harzburgites and dunite-chromite pods display a limited range in Cr/[Cr-i-Al] = 0.32-0.47, which overlaps the data array defined by abyssal peridotites formed beneath ocean ridges. Whole-rock major and trace element data show that the crustal lithologies resemble mid-ocean ridge basalts (MORB) (eg. [La/SmJN-MORB < 1 and high [La/Nb]N-MORB > 1), and may have resulted from a moderate extent (10-20%) of melting of an incompatible element-depleted Iherzolite source with the Marlborough harzburgite as residue. Whole rock Sm-Nd isotopic data for five cogenetic mafic samples define an isochron equivalent to an age of 562 ± 22 Ma (2δ) with an εNd = +8.89 ± 0.15, similar to the estimated Nd isotopic value of the depleted upper mantle at 562 Ma. The second event is represented by tholeiitic and calc alkaline basaltic magmatism interpreted to be of island arc origin. The basalts exhibit high Sr/Nd, Ba/Nb and Th/Yb ratios and excess depletion of high- field strength elements (HFSE) such as Nb, Ta and Ti compared to rare earth elements (REE) of similar incompatibility. These geochemical systematics of the basalts, plus the high Cr/[Cr+Al] values (0.68-0.80) in chromium spinel of the associated gabbros, indicate their formation in an oceanic arc setting. Whole-rock Sm-Nd isotopic data for four cogenetic mafic samples define an isochron equivalent to an age of 380 ± 19 Ma (2δ) with an εNd = +8.07 ±0.1. The third event is represented by tholeiites and alkaline basalts with enriched geochemical signatures (eg. La/Sm > 2 and Nb/Zr > 0.1). They are interpreted to represent products of intraplate volcanism and give a combined whole-rock and mineral Sm-Nd isochron age of 293 ± 35 Ma (2δ) with an εNd = +6.84 ± 0.19. The fourth event is represented largely by andesites, I-type granitoids and spatially associated serpentinised harzburgite. The harzburgite retains a supra-subduction zone signature with Cr/[Cr+Al] = 0.79-0.86 in residual chromium spinel. The andesites and I-type granitoids are low in K and display strong arc signatures (eg. high Ba/Ti, U/Nb and Th/Yb ratios and low Nb/La, Ti/Sc and Nb/Zr) with heavy REE concentrations more depleted than the average N-MORB, typical of oceanic arc magmatism. The I-type granitoids are similar to adakites (high in Al and Sr, low in Y, Nb, Yb and Rb/Sr), characteristic of the melting of young, hot subducting oceanic lithosphere. K-Ar mineral isotopes record a magmatic age of 277 ±7 Ma.
These results depict a dynamic history for the development of the eastern Australian continent from the late Proterozoic through Palaeozoic. This history began with the generation of a late Neoproterozoic major ocean basin (VI magmatism) adjacent to the Australian craton, possibly in response to the fragmentation of Rodinia. The subsequent formation of Gondwanaland led to the subduction of oceanic lithosphere beneath the eastern Australian margin and initiation of the Samfrau orogenic system. Failed subduction and backarc/forearc spreading resulted in an oceanward migration of the volcanic front in the early Cambrian and the development of an island arc system with landward-dipping subduction. Partial cratonisation of the overriding plate to the eastern Australian margin and a shallowing in the angle of the subducting lithosphere from the Cambrian through to the Siluro-Devonian resulted in the gradual movement of the volcanic arc back towards the growing continent (V2 magmatism). This culminated in an Andean-style margin in the late Devonian. Crustal extension was initiated in the late Carboniferous through to early Permian (V3 magmatism) before re-establishment of a convergent margin in the late Early Permian (V4 magmatism).