Sources and evolution of arc magmas inferred from coupled O and Hf isotope systematics of plutonic zircons from the Cretaceous Separation Point Suite (New Zealand)

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Abstract

Coupled O and Hf isotopic compositions of zircons from Early Cretaceous (113–124 Ma) granitoids of the Separation Point Suite (SPS), New Zealand, obtained by cathodoluminescence imaging-guided micro-beam methods (SIMS, LA-ICPMS), are used as a record of evolving magma compositions in a prominent Mesozoic arc system. Eight representative SPS samples from individual plutons in the Nelson and Fiordland regions yield magmatic zircons with initial Hf isotope ratios (expressed in εHf) ranging from − 4 to + 11 (Nelson) and + 5 to + 12 (Fiordland), respectively. Initial Hf isotope ratios of zircons are extremely heterogeneous within individual samples, with the vast majority of values distinctly less radiogenic than depleted mantle at ~ 120 Ma (εHf ~ + 16). O isotope ratios are likewise variable, with δ18Ozircon (SMOW) values of 2–8‰ (Nelson) and 0–7‰ (Fiordland). The within-rock variability in both zircon Hf and O isotope ratios is testimony to open-system processes that operated during magma evolution and zircon crystallisation. Average δ18Ozircon for cores and rims allow constraints to be placed on the O isotopic composition of magmas from which zircon precipitated (δ18Omagma ~ 4–8‰). Elevated δ18Omagma (> 6.5‰) require involvement of 18O-enriched supracrustal material (weathered continental crust or low-T seawater-hydrothermally altered oceanic crust), while δ18Omagma < 5.5‰ imply contribution from a 18O-depleted crustal component. Whole rock Sr isotope, Nb/Ta and Nd/Pb systematics are inconsistent with 18O-depleted slab melts (δ18O ~ 0–6‰) as a source component for SPS magmas. Instead, low δ18O values suggest incorporation of high-T meteoric-hydrothermally altered country rocks similar to those of the Largs terrane presently exposed in northern Fiordland. In diagrams of SiO2 versus inferred δ18Omagma the most primitive SPS samples from the Nelson and Fiordland regions plot close to the expected composition for primitive arc magmas. More evolved granitoids, however, show strongly divergent trends of O isotope composition as a function of SiO2, suggestive of assimilation coupled to fractional crystallisation (AFC) in contrasting crustal environments. Emplacement-level contamination by local crust is supported by age distributions of inherited zircons, which indicate a predominance of Palaeozoic and Mesozoic zircons in Nelson and Fiordland granitoids, respectively.

Introduction

A general consensus prevails among geochemists and petrologists that arc magmas generated along oceanic and continental margins receive variable contributions from mantle, crustal and subducted reservoirs (e.g. Hildreth and Moorbath, 1988). However, the exact proportions and mechanisms by which these heterogeneous components are involved in arc plutonic and volcanic systems remain elusive. Some of the ambiguity stems from using the whole rock geochemical approach, because such data generally represent the end-product of a complex petrogenetic evolution involving multiple sources and processes. The problem of resolving the respective roles of mantle- and crustal-derived end-members in igneous petrogenesis is highlighted by the Separation Point Suite (SPS), a major crustal component of the South Island of New Zealand, which was formed by subduction along the Gondwana margin during the Early Cretaceous. Previous studies explained whole rock Sr and Nd isotopic data in terms of 1) interaction between old continental crustal material and melts from MORB-source mantle (Pickett and Wasserburg, 1989) or 2) partial fusion of a mafic underplate that was underthrust at the base of a thickened arc complex (McCulloch et al., 1987, Muir et al., 1995, Muir et al., 1998, Tulloch and Kimbrough, 2003). Most recently, it has been argued that Hf isotope ratios of SPS granitoids, being significantly more radiogenic than continental crust, but more evolved than the hypothesized mantle-derived mafic underplate, require additional contributions from an enriched mantle source (EMI), in line with calculated whole rock δ18O values of ~ 5–8‰ (Tulloch et al., 2006).

To explore the origin of the SPS, we have undertaken a coupled O and Hf isotope study of magmatic zircon from 8 plutonic rocks using state-of-the art micro-analytical methods and aided by detailed cathodoluminescence (CL) imaging. The information retrieved at the mineral scale is integrated with published whole rock geochemical and isotopic information. Because O isotopes are sensitive to water–rock interaction, they serve as fingerprints for material that had experienced weathering or hydrothermal alteration. Consequently, O isotope systematics of plutonic zircons aid in distinguishing between unaltered, mantle-derived and recycled components (e.g. Valley, 2003, Kemp et al., 2007, Scherer et al., 2007). Likewise, Hf isotopes can be used as powerful tracers for the involvement of primitive (i.e. mantle-derived) and continental crustal sources (e.g. Griffin et al., 2002, Kinny and Maas, 2003, Yang et al., 2007).

The present study is specifically directed towards resolving the influence of source characteristics (slab, mantle wedge, lithosphere) versus emplacement level processes (assimilation of country rock, magma mixing). Since the Separation Point Suite is a major crustal component with distinct geochemical compositions (adakitic, similar to the Tonalite–Trondhjemite–Granodiorite or TTG series), our new data have important ramifications for the genesis of arc-related plutonic rocks, especially with reference to those of adakitic, TTG and I-type affinity.

Preliminary results have appeared in abstract form (Bolhar et al., 2006).

Section snippets

Geology

Pre-Mid Cretaceous basement rocks of the South Island of New Zealand are grouped into the Western and Eastern provinces, separated by the geologically complex Median Tectonic Zone (Bradshaw, 1989). The provinces, in turn, consist of a number of fault-bounded terranes, each with its own geologic history (Wandres and Bradshaw, 2005). The Western Province is composed of Palaeozoic metasedimentary and metavolcanic rocks, which are intruded by Devonian, Carboniferous and Early Cretaceous granitoids (

Analytical techniques

LA-ICPMS U–Pb ages and trace element analyses for 20–40 zircon grains from each of the studied samples were reported in a previous study (Bolhar et al., 2007). Guided by cathodoluminescence (CL) imaging, suitable grains were chosen for micro-beam analysis on the basis of textural make-up and size. Between 3 and 7 representative grains per sample were used for SIMS O isotope analysis, with a focus on magmatic cores and rims and inherited cores. The objective was to examine intra-grain

Cathodoluminescence imaging

In this study, the innermost zones within zircons are referred to as cores, irrespective of whether they are inherited, based on dates that are significantly older than the supposed crystallisation age of the host rock, or are of similar age and, hence, probably represent an early phase of the same magmatic event. Rims are defined as representing material enclosing the inner portions. For O and Hf isotope analysis, emphasis was placed on selecting a wide variety of zircons with differing sizes,

Intra-grain variability — a record of evolving melt composition

Isotopic variability between and within grains can arise from a variety of processes and entails contributions from distinct compositional components during magma differentiation. This study is one of a few to date to utilize crystallisation temperatures inferred from Ti concentrations as an additional constraint to investigate the thermal history during zircon growth (Watson and Harrison, 2005). This novel approach therefore enables us to examine coupled compositional, isotopic and thermal

Final remarks

The SPS granitoids exhibit zircon Hf and O isotope values that are systematically more mantle-like than any observed by Kemp et al. (2007) for Palaeozoic granitic suites exposed in south-eastern Australia. Thus, arc magmatism along the Mesozoic continental margin of Gondwana, at least in New Zealand, involved much less reworking of older supracrustal material and a greater proportion of net crustal growth than had been the case during the Palaeozoic. In addition, the conclusion that interaction

Acknowledgements

This research was funded through Marsden grant UOC0508 to JWC and SDW. RB acknowledges financial support from the University of Canterbury, in the form of a postdoctoral fellowship. JMP acknowledges funding by a University of Otago Research Grant. Michelle Herd, Roland Maas and, in particular, Lorraine Paterson provided very valuable assistance during ICPMS and CL work. Rob Spiers, Jennifer Jackson, Kerry Swanson and Sacha Baldwin-Cunningham are thanked for help with mineral separation. The

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