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Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of *9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive e Nd values (þ6.9 to þ7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee -Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium-to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee -Narromine Volcanic Belt have ages that cluster in the 470 -460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calcalkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley -Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee -Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley -Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian -Devonian opening of the Hill End Trough. Phase 3 magmatism in the Macquarie Arc is represented by a widespread but relatively small volume magmatic event, dominated by shallow intrusive rocks of the Copper Hill Suite, emplaced in the Eastonian -Bolindian, between 456 and 441 Ma. These distinctive porphyritic dacites and associated holocrystalline diorites and granodiorites show medium-K calc-alkaline compositions, and their emplacement was intimately linked to an episode of regional uplift, erosion and limestone deposition in the Junee -Narromine Volcanic Belt and western Molong Volcanic Belt. Phase 4 magmatism extended from late Eastonian or Bolindian until Early Silurian time, and was dominated by relatively evolved (compared with Phase 2 lavas) shoshonitic lavas until the end of the Bolindian and porphyries in the Early Silurian. Collision-related shut-down of the arc, and initiation of arc extension and dismemberment, occurred around 438 Ma in the latest Ordovician. Post-arc magmatism during the Early Silurian is represented by high-Th, high-Nb lavas of the shoshonitic Nash Hill Vo...

Cover sheet Cover sheetTitle Re-evaluation of contact relationships between the Ordovician volcanic belts and the quartz-rich turbidites of the Lachlan Orogen

The consensus view is that the O 2 concentration of the Archean atmosphere was very low and that it rose to its present level of 21% in a series of steps, two of which dwarf the others in importance. The first, known as the Great Oxidation Event, occurred at $2.4 Ga. It involved an increase in the relative abundance of O 2 , which has been estimated at three orders of magnitude, and it is important because it led to the first surface weathering. The second, although less important in relative terms, involved the addition of 9 Â 10 17 kg of O 2 to the atmosphere, at least ten times as much as that required to produce the Great Oxidation Event. Its importance lies in the fact that it correlates with the rise of animals in the Ediacaran and Early Cambrian periods. Although it is widely accepted that an increase in atmospheric O 2 facilitated the appearance of animals at $575 Ma, followed by the Cambrian Explosion $50 Myr later, the cause of this increase remains controversial. We show that the surge in the O 2 level near the Precambrian-Cambrian boundary correlates with major episodes of continent-continent collision associated with Gondwana's amalgamation, including convergence between East and West Gondwana, which produced the 8000-km-long Transgondwanan Supermountains. The eroded roots of these mountains include the oldest lawsonite-bearing blueschists and eclogites, and ultra high-pressure metamorphic rocks. The sudden appearance of these lowthermal gradient, high-pressure metamorphic rocks implies that the Gondwanan orogenic zones were cooler and stronger than those associated with the assembly of earlier supercontinents and therefore capable of supporting higher mountains.There is a log-linear relationship between relief and erosion rate, and a linear relationship between sedimentation rate and organic C burial. Taken together these two relationships imply a log-linear relationship between relief and C sequestration. We suggest that the Gondwanan supermountains were higher than those produced during the assembly of earlier supercontinents and that rapid erosion of these mountains released a large flux of essential nutrients, including Fe and P, into the rivers and oceans, which triggered an explosion of algae and cyanobacteria. This, in turn, produced a marked increase in the production rate of photosynthetic O 2 . Rapid sedimentation during this period promoted high rates of burial of biogenic pyrite and organic matter generated during photosynthesis so that they could not back react with O 2 , leading to a sustained increase in atmospheric O 2 .