Evolution of the Multiply Metamorphosed Broughton Arm Meta-Peridotite, and Implications for the Wider Fiordland Region

Dwight, Thomas (Tom)

The Broughton Arm Meta-Peridotite is thought to represent a multiply metamorphosed ultramafic cumulate intrusion within Central Fiordland. This unit was first recognised by the QMAP Fiordland mapping program but has not been studied until now. The ultramafic body outcrops as a two oblate lobes trending north-south, encased in a thin hornblendite rim. It is situated within Early Paleozoic metasedimentary rocks that are thought to be a part of Deep Cove Gneiss (Takaka Terrane). Evidence for a cumulate origin is seen through bulk rock analysis, which return Mg# values of 78 – 79, far below that of mantle peridotite values. The primary metamorphic mineral assemblage (M1) includes olivine, enstatite and magnetite, and is overprinted by tremolite, chlorite, serpentinite, ± magnesite ± talc ± smectite ± anthophyllite, these conditions reflect ~amphibolite facies conditions. Extremely variable olivine Mg# (78 – 93) across the outcrop suggests it was also lithologically heterogeneous. The hydrous mineralogy likely reflect a fluid-rich metamorphic event (M2). The age of M1 is unknown, but it is tentatively correlated with a metamorphic event in the Carboniferous that has been documented from published titanite grains within the Deep Cove Gneiss in nearby Vancouver Arm. The M2 event may be associated with the extensional tectonics that lead to the rifting of Zealandia from Gondwana, in the amphibolite facies adjacent (~200 m) Doubtful Sound Shear Zone. The presence of this metamorphosed ultramafic unit provides evidence for (probable) Paleozoic high-temperature melting of the mantle under Fiordland.

101 pages A4

2016Dwight

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