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)
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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