Abstract submitted for AGC meeting Townsville July 1999

GEOCHEMISTRY AND GEOCHRONOLOGY OF THE MARLBOROUGH OPHIOLITE: IMPLICATIONS FOR THE TECTONIC HISTORY OF THE NORTHERN NEW ENGLAND FOLD BELT

 

Michael Bruce, Yaoling Niu, Terrence Harbort and Rodney Holcombe

Department of Earth Sciences, The University of Queensland, St Lucia, Queensland 4072

 

The New England Fold Belt (NEFB) extends for ~2000 km along the eastern margin of Australia and is a tectonic collage of mid-Palaeozoic subduction/accretion elements, late Palaeozoic extensional basins and structures all overprinted by a major Permo-Triassic contractional event. At the northern end of the NEFB thrusting associated with this latest event has exposed a dismembered ophiolite (Marlborough ophiolite), the largest (~700 km2) complex of this type known in Australia. The ophiolite is tectonically interleaved with rocks of the mid-Palaeozoic accretionary complex (~370-340 Ma). An ophiolitic component has previously been recognised within these accretionary units to the south. An apparent ophiolitic component also crops out on South Percy Isle which appears to be separated from the Marlborough ophiolite by the Stanage Fault Zone, an approximate north-northeast trending oblique shear zone with dextral strike slip offset. Previous interpretations have been that these are elements of oceanic crust underplated by basal accretion during subduction and then exhumed during subsequent events.

 

Geochemical and age data suggest that the Marlborough ophiolite is a fragment of normal oceanic lithosphere formed at an ancient oceanic ridge, which is at least 170 Ma older than the accretionary complex, and has been intruded at least once by mafic dykes with island arc signatures. These observations are interpreted to indicate that the eastern margin of Australia evolved from an Island arc/Back-arc system to an Andean-style convergent margin.

Despite the structural complications and metamorphic overprinting, residual harzburgitic mantle rocks (partially to entirely serpentinised) and crustal lithologies such as basalts and gabbroic cumulates are readily identified in the ophiolite. Unaltered cores of relic igneous chromium spinel from both the harzburgites and dunite-chromite pods display a limited range in Cr/[Cr+Al] = 0.3-0.4, which overlaps the data array defined by abyssal peridotites formed beneath ocean ridges. Within the partially serpentinised harzburgites, igneous olivine has a composition of Fo90-91 and orthopyroxene ranges in Al2O3 from 2.9 to 3.7 wt%. Minor clinopyroxene has a Cr2O3 content of 0.9-1.3 wt%. All these are typical of normal oceanic peridotites. The crustal cumulates and entrained pockets of primary liquid compositions of the ophiolite contain geochemical signatures of depleted average N-MORB. Whole-rock and mineral separates from one of the gabbros give a Sm-Nd isochron age of 561 + 23 Ma (2s error) and a e Nd = 8.7 + 0.2. Some apparently younger (unconstrained geochronologically) mafic dykes, that both cut the harzburgites and form tectonically emplaced slices within the fault-bounded ophiolite blocks, show Island Arc geochemical signatures. Still younger [251 + 78 Ma (2s ), e Nd = 6.8 + 0.3 - whole rock Sm-Nd isochron] mafic dykes with ‘enriched’ incompatible trace element signatures also exist and appear to be broadly coeval with the Permo-Triassic contractional event. The enriched signatures may have resulted from contamination by continental crustal material.

 

Preliminary studies of ultramafic lithologies from elsewhere in the northern NEFB indicate diverse origins. Residual spinels in serpentinites of the accretionary North D’Aguilar block have Cr/[Cr+Al] = 0.38-0.45, which appears consistent with a spreading ridge origin. Despite variable alteration high chrome spinels (Cr/[Cr+Al] = 0.6) are recorded from serpentinites along the Yarrol fault. Serpentinite domes from the Bowen Basin contain Cr spinel with Cr/[Cr+Al] = 0.55. Serpentinised harzburgites, previously assumed to be displaced remnants of the Marlborough ophiolite, crop out on South Percy Isle, ~150 km north of Rockhampton. These harzburgites contain residual spinels with Cr/[Cr+Al] @ 0.8, suggesting that they may have formed in a supra subduction zone environment, and are therefore of a different origin than that of the Marlborough ophiolite. This observation suggests that the Stanage Fault Zone may have had a long history, and is tectonically far more significant than a simple transcurrent displacement.