Tectonic evolution of the Fiordland convergent Gondwana margin

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Scott, James; Palin, J. Michael; Cooper, Alan
Holding Date 11 October 2008

The composite Carboniferous-Jurassic magmatic arc belt of the Median Tectonic Zone/ Median Batholith/ Tutoko Complex comprises the crustal framework of Eastern Fiordland (South Island, New Zealand) and occupies a thin zone between the Gondwana continental margin (Western Fiordland) and its accretionary terranes. However, there is little agreement on the origin or evolution of this belt. With respect to Western Fiordland, it has been interpreted to have formed allochthonously or autochthonously. Rocks within this area are inferred to have over-thrust, under-thrust or obliquely collided with Gondwana, or to have formed in place. These mutually exclusive scenarios have been determined from geochronological and geochemical aspects of plutonic rocks.
P-T constraints and detrital and metamorphic zircon data from paragneisses in both Eastern and Western Fiordland were used to investigate the relationship of Eastern Fiordland to the Gondwana continental margin. Detrital zircon spectra from the previously undescribed Mt. Crescent and Borland Paragneiss units from Eastern Fiordland record the influx of Western Fiordland detritus no earlier than the Middle Jurassic. Quantitative P-T estimates on metamorphic assemblages coupled with LA-ICP-MS analyses of low Th/U overgrowths on detrital zircon grains demonstrate metamorphism of the Borland Paragneiss under amphibolite facies conditions (4.2 + 0.6 kbar, 625 + 25°C) at 145 + 3 Ma. The Mt. Crescent Paragneiss was also metamorphosed in the amphibolite facies (570 + 14°C, 5.2 + 0.8 kbar), but detrital zircon grains lack a resolvable metamorphic overprint. However, the Russet Paragneiss, which lies across the Grebe Fault in Western Fiordland, was deposited no earlier than Early Ordovician, metamorphosed at 7.5 + 0.6 kbar, 633 + 25°C at 349 + 12 Ma and exhibits no evidence for subsequent metamorphism in the Jurassic.
These results imply that (1) the Eastern Fiordland arc and Western Fiordland Gondwana continental margin were juxtaposed in post-Jurassic times. (2) The Grebe Fault, which separates geological blocks, is a major terrane-bounding structure. (3) Mesozoic fore-arc accretionary sedimentary terranes lying east of Fiordland and comprising much of the Zealandia continent were first accreted to the Eastern Fiordland arc before faulted against Gondwana. (4) Plutonic rocks with shared characteristics that likely reflect common sources in the deep crust and/or lithosphere provide an incomplete record of the evolution of this Mesozoic convergent margin.