Geochronology and geochemistry of a Mesozoic magmatic arc system, Fiordland, New Zealand

Abstract

The Median Tectonic Zone in Eastern Fiordland, SW New Zealand, comprises a tectonically disrupted belt of Mesozoic magmatic arc rocks related to subduction along the palaeo-Pacific margin of Gondwana. New ion microprobe (SHRIMP) U-Pb zircon ages confirm that the bulk of the plutonic rocks in eastern Fiordland range from Mid-Jurassic to Early Cretaceous (168-137 Ma) in age. Carboniferous age granitoids occur in SW Fiordland, along the western side of, and within the zone. Triassic plutonic rocks appear to be restricted to the eastern side of the zone. The Mid-Jurassic-Early Cretaceous igneous rocks (collectively referred to as the Darran Suite) are cut by several plutons of Na-rich granitoid (Separation Point Suite) that give ages of c. 124 Ma, slightly older than equivalent rocks in the NW part of the South Island. Early Cretaceous granulite facies orthogneisses (126-119 Ma) in western Fiordland (Western Fiordland Orthogneiss) are considered to be the lower crustal equivalent of the Separation Point plutons.The majority of the Darran Suite rocks are I-type, hornblende-bearing calc-alkaline igneous rocks, most likely derived from melting in the mantle wedge above a subducting slab of oceanic lithosphere. In contrast, the Separation Point-type plutons are Na-rich, alkali-calcic granitoids with high concentrations of Sr (typically >500 ppm and up to 1000 ppm) and low concentrations of Y (c5 ppm) and heavy REE (<10 times chondritic). Isotopic compositions are primitive, with 87 Sr/ 86 Sr initial ratios of c. 0.7038, and Nd values of c. +3 at 120 Ma. Their geochemistry is consistent with melting of a mafic protolith of garnet amphibolite mineralogy. Mafic Darran Suite rocks have the appropriate chemical and isotopic compositions to generate the Western Fiordland Orthogneiss and the higher level Separation Point type plutons. We suggest that the sudden appearance of large volumes of Na-rich magma during the Early Cretaceous was triggered tectonically, perhaps by thrusting of the Median Tectonic Zone arc beneath western New Zealand. Melting of basal arc underplate at depths of >40 km would then have generated Na-rich granitoids, leaving residues of garnet+clinopyroxene+amphibole.

Plutonic rocks of the Median Batholith in eastern and central Fiordland, New Zealand: Field relations, geochemistry, correlation, and nomenclature

“…it includes the easternmost paleozoic Stype granitoids in Fiordland. previous work has suggested autochthonous (blattner 1991; Mortimer et al 1999a;wandres & bradshaw 2005), para-autochthonous , and allochthonous (Muir et al 1998;powell 2006) relationships across this boundary within the Fiordland basement. in southern Fiordland this boundary is marked by the grebe Mylonite Zone (Scott 2008), and on Stewart island it is marked by the Escarpment Fault and gutter Shear Zone (Allibone & Tulloch 2004,2008.…”

“…Representative type localities are nominated for each plutonic map unit, chosen to reflect their internal character. Key characteristics of each unit, such as constituent lithologies and accessory mineral assemblages, are summarised in Table 1. plutons are described in the order oldest to youngest, based on field relationships and U-Pb zircon ages Muir et al 1998;Ramezani & Tulloch 2009;Tulloch et al in press). only the relationships of a particular pluton to older rocks are generally described to save repetition.…”

“…Each of these units was inferred to include numerous plutons, although these were rarely mapped at the time. These large-scale units include the western province (landis & coombs 1967), the Separation point Suite (Tulloch 1983(Tulloch ,1988, the Median Tectonic Zone (MTZ) (bradshaw 1993; Kimbrough et al 1993Kimbrough et al ,1994Muir et al 1995Muir et al , 1998, the Median Suite (Mortimer & Tulloch 1996;Tulloch & Kimbrough 2003), the darran Suite (Muir et al 1998), the Median batholith (Mortimer et al 1999a), and the Tutoko complex (wandres & bradshaw 2005). Revised definitions of the plutonic suites are presented below.…”

“…u-pb zircon TiMS, SHRiMp, and lA-icp-MS geochronology has been used by Kimbrough et al (1994), Muir et al (1998), Scott (2008), and to examine the timing of plutonism in the eastern half of Fiordland, and to establish correlations with plutonic and volcanic rocks in adjacent terranes. Mesozoic magmatism in eastern Fiordland ranges in age between c. 230 and 116 Ma, with distinct pulses at 230-224, 168-128, and 125-116 Ma (Kimbrough et al 1994;Muir et al 1998;Ramezani & Tulloch 2009).…”

“…Mesozoic magmatism in eastern Fiordland ranges in age between c. 230 and 116 Ma, with distinct pulses at 230-224, 168-128, and 125-116 Ma (Kimbrough et al 1994;Muir et al 1998;Ramezani & Tulloch 2009). isolated occurrences of c. 340 Ma carboniferous plutonic rocks in eastern Fiordland may represent fragments of country rock to the Mesozoic plutonic rocks which dominate this part of Fiordland .…”