The Tempo of Continental Arc Construction in the Mesozoic Median Batholith, Fiordland, New Zealand

Abstract

We investigate the temporal record of magmatism in the Fiordland sector of the Median Batholith (New Zealand) with the goal of evaluating models for cyclic and episodic patterns of magmatism and deformation in continental arcs. We compare 20 U-Pb zircon ages from >2300 km2 of Mesozoic lower and middle crust of the Western Fiordland Orthogneiss to existing data from the Median Batholith to: (1) document the tempo of arc construction, (2) estimate rates of magmatic addition at various depths during arc construction, and (3) evaluate the role of cyclical feedbacks between magmatism and deformation during high and low magma addition rate events. Results from the Western Fiordland Orthogneiss indicate that the oldest dates are distributed in northern and southern extremities: the Worsley Pluton (123–121 Ma), eastern McKerr Intrusives (128–120 Ma), and Breaksea Orthogneiss (123 Ma). Dates within the interior of the Western Fiordland Orthogneiss (Misty and Malaspina Plutons, western McKerr Intrusives) primarily range from 118 to 115 Ma and signify a major flux of mafic to intermediate magmatism during which nearly 70% of the arc root was emplaced during a brief, ~3 m.y., interval. The spatial distribution of dates reveals an inward-focusing, arc-parallel younging of magmatism within the Western Fiordland Orthogneiss during peak magmatic activity. Coupled with existing data from the wider Median Batholith, our data show that Mesozoic construction of the Median Batholith involved at least two high-flux magmatic events: a surge of low-Sr/Y plutonism in the Darran Suite from ca. 147 to 136 Ma, and a terminal surge of highSr/Y magmatism in the Separation Point Suite from 128 to 114 Ma, shortly before extensional collapse of the Zealandia Cordillera at 108–106 Ma. Separation Point Suite magmatism occurred at all structural levels, but was concentrated in the lower crust, where nearly 50% of the crust consists of Cretaceous arc-related plutonic rocks. Existing isotopic data suggest that the flare-up of high-Sr/Y magmatism was primarily sourced from the underlying mantle, indicating an externally triggered, dynamic mantle process for triggering the Zealandia high–magma addition rate event, with only limited contributions from upper plate materials.