Tracking the influence of a continental margin on growth of a magmatic arc, Fiordland, New Zealand, using thermobarometry, thermochronology, and zircon U‐Pb and Hf isotopes

Abstract

Geothermobarometric, radiogenic isotopic and thermochronologic data are used to track the influence of an ancient continental margin (Western Province) on development of an adjacent Carboniferous‐Cretaceous magmatic arc (Outboard Median Batholith) in Fiordland, New Zealand. The data show a record of complicated Mesozoic Gondwana margin growth. Paragneiss within the Outboard Median Batholith is of Carboniferous to Jurassic age and records burial to middle crustal depths in Late Jurassic–Early Cretaceous during subduction‐related plutonism and arc thickening. In contrast, Western Province metasedimentary rocks in the area of study immediately west of the Outboard Median Batholith are Late Cambrian–Early Ordovician in age, recrystallized at the amphibolite facies in the Late Devonian–Early Carboniferous and exhibit no evidence for Mesozoic textural or isotopic reequilibration. A phase of deformation, between 128 and 116 Ma deformed, exhumed, and cooled the Outboard Median Batholith to greenschist facies temperatures, while large parts of the Western Province underwent ≥9 kbar metamorphic conditions. Zircon grains from Mesozoic inboard plutons are isotopically more evolved (ɛHf(t) = +2.3 to +4.0) than those in the Outboard Median Batholith (ɛHf(t) = +9.4 to +11.1). The contrasting zircon Hf isotope ratios, absence of S‐type plutons or Proterozoic–Early Paleozoic inherited zircon, and the apparent absence of Early Paleozoic metasedimentary rocks indicates that the Outboard Median Batholith is unlikely to be underlain by the Western Province continental lithosphere. The new data are consistent with the Outboard Median Batholith representing an allochthonous (although not necessarily exotic) arc that was juxtaposed onto the Gondwana continental margin along the intervening Grebe Mylonite Zone.

Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny

“…By contrast, granulites in magmatic arc roots commonly witnessed a high geothermal gradient of N 20°C/km (Bohlen, 1987;Ellis, 1987;Harley, 1989;Bohlen, 1991;Ringuette et al, 1999;Müntener et al, 2000;Bhowmik et al, 2005;Brandt et al, 2006;Zhang et al, 2013;Zhao & Zhai, 2013). In general, plate convergence and magma accretion resulting in crustal thickening and growth, and associated HT/UHT granulite-facies metamorphism and anatexis may be one of the most important processes in subduction-related magmatic arcs (e.g., Gibson et al, 1988;Brown, 1996;Clarke et al, 2000;Collins, 2002aCollins, , 2002bHollis et al, 2003; Hyndman et al, 2005;Currie & Hyndman, 2006;Klepeis et al, 2007;Scott et al, 2009;Stowell et al, 2010;Yin et al, 2014).…”

Zetland Diorite, Karamea Batholith, west Nelson: field relationships, geochemistry and geochronology demonstrate links to the Carboniferous Tobin Suite

“…Calculated oHf (t) values for the c. 343 Ma William Granite (Takaka Terrane) show a larger range of more primitive values ('4.1 to '9.9; Scott et al 2009) than the range of primitive oHf(t) exhibited in Zetland Diorite zircon (Buller Terrane; '0.9 to '5.8; R. Turnbull unpublished data). The large range in oHf(t) for the Zetland Diorite is in contrast to a restricted range in zircon d 18 O ('7.4 to '7.8; R. Turnbull unpublished data), which implies a heterogeneous mantle and/or lower crustal mafic source and very limited Greenland Group supracrustal input.…”

Carboniferous metamorphism and partial melting of the Greenland Group in the Jackson River valley, south Westland

“…It has also affected parts of central Fiordland (Fig. 1) (Ireland & Gibson 1998;Daczko et al 2009;Scott et al 2009a) and northern Fiordland (Scott et al 2009b). Quantitative P-T estimates for the Fiordland Early Carboniferous metamorphic fabrics yielded values of c. 6-8 kbar and ≤ 650°C (Ireland & Gibson 1998;Scott et al 2009a,b).…”

“…In a situation in which the laser drills right through a zoned grain, multiple dates can be determined from a single spot analysis (e.g. Scott et al 2009a). This technique was used to date the overgrowths on zircon grains.…”