Autochthonous inheritance of zircon through Cretaceous partial melting of Carboniferous plutons: the Arthur River Complex, Fiordland, New Zealand

作者：Tulloch Andrew J^*; Ireland Trevor R; Kimbrough David L; Griffin William L; Ramezani Jahandar

来源：Contributions to Mineralogy and Petrology, 2011, 161(3): 401-421.

DOI：10.1007/s00410-010-0539-6

• U-Pb geochronology
• Gabbro-diorite
• Inheritance
• Resetting
• Partial melting
• Fiordland

TIMS and SHRIMP U-Pb analyses of zircons from Milford Orthogneiss metadiorite (P = 1-1.4 GPa; T a parts per thousand yen 750A degrees C) of the Arthur River Complex of northern Fiordland reveal a bimodal age pattern. Zircons are predominantly either Paleozoic (357.0 +/- A 4.2 Ma) and prismatic with oscillatory zoning, or Cretaceous (133.9 +/- A 1.8 Ma) and ovoid with sector or patchy zoning. The younger age component is not observed overgrowing older grains. Most grains of both ages are overgrown by younger Cretaceous (similar to 120 Ma) metamorphic zircon with very low U and Th/U (0.01). We interpret the bimodal ages as indicating initial igneous emplacement and crystallisation of a dioritic protolith pluton at similar to 357 Ma, followed by Early Cretaceous granulite-facies metamorphism at similar to 134 Ma, during which a significant fraction (similar to 60%) of the zircon grains dissolved, and subsequently reprecipitated, effectively in situ, in partial melt pockets. The remaining similar to 40% of original Paleozoic grains were apparently not in contact with the partial melt, remained intact, and show only slight degrees of Pb loss. Sector zoning of the Cretaceous grains discounts their origin by solid state recrystallisation of Paleozoic grains. The alternative explanation-that the Paleozoic component represents a 40% inherited component in an Early Cretaceous transgressive dioritic magma-is considered less likely given the relatively high solubility of zircon in magma of this composition, the absence of 134 Ma overgrowths, the single discrete age of the older component, equivalent time-integrated Hf-177/Hf-176 compositions of both age groups, and the absence of the Cambrian-Proterozoic detrital zircon that dominates regional Cambro-Ordovician metasedimentary populations. Similar bimodal Carboniferous-Early Cretaceous age distributions are characteristic of the wider Arthur River Complex; 8 of 12 previously dated dioritic samples have a Paleozoic component averaging 51%. Furthermore, the age and chemical suite affinity of these and several more felsic rocks can be matched with those of the relatively unmetamorphosed Carboniferous plutonic terrane along the strike of the Mesozoic margin in southern Fiordland, also supporting the in situ derivation of the Carboniferous "inherited" component.

• 出版日期2011-3