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A regional petrographic reconnaissance of psammitic and pelitic rocks in the Otago Schist, New Zealand, has revealed the presence of garnet (ªgros-salspiteº with typical rim composition almandine 41 , spessartine 25 , grossular 33 , pyrope 1 ) and biotite in 37 new samples, more than doubling the previously known number. A new garnet±biotite±albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-known biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet±biotite±albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple, continuous, progressive Jurassic regional metamorphism in Otago. The structurally higher (lower grade) boundary of the zone coincides in at least three places with previously mapped regional shear zones. The structurally lower (expected higher grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.

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Oceanic plateaus and continental flood basalts, collectively referred to as large igneous provinces (LIPs), represent the most voluminous volcanic events on Earth. In contrast to continental LIPs, relatively little is known about the surface and internal structure, range in age and chemical composition, origin, and evolution of oceanic plateaus, which occur throughout the worlds oceans [e.g., Mahoney and Coffin, 1997]. One of the major goals of the R/V Sonne SO168 ZEALANDIA expedition (depart Wellington, 3 December 2002, return Christchurch, 15 January 2003) was to investigate the Hikurangi oceanic plateau off the east coast of New Zealand.

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Collision of a young, buoyant plateau fragment with an active continental margin can lead to subduction cessation, but can it also trigger continental breakup? It has been postulated that the collision of the Hikurangi Plateau with the Gondwana margin at ~110 Ma ago, caused subduction to cease, large-scale extension and ultimately breakoff of the Zealandia microcontinent from West Antarctica through seafloor spreading starting at circa 90 Ma. Here we report new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data for Late Cretaceous (99-69Ma) volcanism from Zealandia. Four geographically-separated provinces of intraplate magmatism formed during this time interval: 1) Hikurangi Seamount Province *Manuscript Click here to view linked References

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This paper provides a regional-scale background for understanding gold-mineralising processes in the Otago Schist during the Cretaceous. At this time the schist belt was in the latter stages of formation as an accretionary complex with 2000 km strike length on the Pacific margin of Gondwana. The Otago Schist is interpreted as an exhumed accretionary wedge of structurally stacked clastic metasedimentary rocks with minor metabasic rocks. Metamorphic grade reached upper greenschist facies. Gold and other related elements were mobilised from the metasedimentary rocks during metamorphism, and these elements contributed to the high levels of orogenic gold endowment (>18 million ounces in the schist belt. Mesozoic Gold deposits were emplaced in two distinct pulses, one at the beginning of the Early Cretaceous (~140-135 Ma) and the other at the end of the Early Cretaceous (~112-100 Ma). These mineralising pulses were driven by regional tectonic events that may have involved episodic underplating of subducting material, and/or subduction of spreading ridges. The earlier event was most closely associated with metamorphic processes that mobilised gold from the accreted metasediments during convergent tectonics, and appears to have been the more economically significant. The later event took place as accretionary processes ceased and the Zealandia portion of Gondwana began to undergo intracontinental rifting.