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)

摘要

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 epsilon(Hf)) 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 similar to 120 Ma (epsilon(Hf)similar to+ 16). O isotope ratios are likewise variable, with delta O-18(zircon)(SMOW) values of 2-8%omicron (Nelson) and 0-7%omicron (Fiordland). The within-rock variability in both zircon Hf and 0 isotope ratios is testimony to open-system processes that operated during magma evolution and zircon crystallisation. Average delta O-18(zircon) for cores and rims allow constraints to be placed on the O isotopic composition of magmas from which zircon precipitated (delta O-18(magma)similar to 4-8%omicron). Elevated delta O-18(magma) (>6.5%omicron) require involvement of O-18-enriched supracrustal material (weathered continental crust or low-T seawater-hydrothermally altered oceanic crust), while delta O-18(magma)<5.5%omicron imply contribution from a O-18-depleted crustal component. Whole rock Sr isotope, Nb/Ta and NdfPb systematics are inconsistent with 180-depleted slab melts (618O similar to 0-6%omicron)as a source component for SPS magmas. Instead, low delta O-18(magma) 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 delta O-18(magma) 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. (c) 2008 Elsevier B.V. All rights reserved.