Masters Thesis

Triggering mechanisms for a magmatic flare-up of the lower crust in Fiordland, New Zealand, from U-Pb zircon geochronology and O-Hf zircon geochemistry

The exhumed root of a Triassic to Cretaceous continental arc in Fiordland, New Zealand records a history of episodic, subduction-related magmatism that culminated in a high magma addition rate (MAR) event during which much of the plutonic arc root was emplaced from 125-115 Ma. To evaluate the triggering mechanism(s) associated with this high MAR event samples were collected for paired oxygen and lutetium-hafnium analyses. Sampling spans both the Inboard and Outboard Median Batholith and covers 150 km parallel and 60 km perpendicular to the paleo-arc axis. We report over 200 SIMS zircon O analyses and over 500 LA-MC-ICP-MS Lu-Hf analyses from 27 samples. In addition, we report, 6 new LA-SF-ICP-MS 206Pb/238U dates. Inboard magmatism took place between 134 to 115 Ma with the high-flux event occurring from 125-115 Ma during emplacement of the Western Fiordland Orthogneiss (WFO). Zircon δ18O SIMS values for the Inboard Median Batholith range from 5.2-6.3‰ with 90% of analyses overlapping mantle-like values (5.3 ± 0.6‰; 2SD). LA-MC-ICP-MS results yield initial εHf (Zrn) values ranging from -2 to + 11 with a mean of +4. We investigate possible relative contributions of supra-crustal material during the arc flare-up through mass balance calculations assuming the following mean zircon-normalized δ18O values: mantle (5.3 ‰), supra-crustal rocks of the Deep Cove Gneiss (DCG) (8.9 ‰), and putative underthrusted Darran Suite (4.2‰). Results from binary mass balance mixing of DCG with mantle magma yield values ranging from 0-21% supra-crustal input during the high MAR event. Together, the homogeneity in δ18O (Zrn) and their mantle-like values, the mafic major element geochemistry, and lack of zircon inheritance observed in the WFO suggests that the high MAR event was triggered by mantle melting, likely resulting from changes in subduction zone dynamics, with only minor input from supra-crustal sources (20%).

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