TRIGGERING MECHANISMS FOR A MAGMATIC FLARE-UP IN THE LOWER CRUST OF A CONTINENTAL ARC, FIORDLAND, NEW ZEALAND

The Jurassic to Cretaceous continental arc exposed in Fiordland, New Zealand records a history of episodic, subduction-related magmatism that culminated in a high-flux magmatic event during which much of the plutonic arc root was emplaced from 124-115 Ma. To evaluate the triggering mechanism(s) associated with this high-flux event, we report 7 new zircon ²⁰⁶Pb/²³⁸U ages and >200 zircon O-isotope analyses from 27 samples. Sampling spans both the Inboard and Outboard Median Batholith and covers 150 km parallel and 60 km perpendicular to the paleo-arc axis.

Results indicate that low-flux magmatism from 170 to 135 Ma in the Outboard Median Batholith was characterized by zircon d¹⁸O values ranging from +3.5 to +5.2‰. Inboard magmatism took place between 134 to 115 Ma with the high-flux event occurring as two pulses from 124-120 and 118-115 Ma during emplacement of the Western Fiordland Orthogneiss (WFO). Zircon d¹⁸O values for the Inboard Median Batholith range from +5.2 to +6.3‰ with >90% of analyses overlapping mantle-like values (5.3 +/- 0.6‰). We propose two possible hypotheses to explain these results: 1) high-flux magmatism resulted from mantle wedge melting, possibly enhanced by delamination, with minimal supracrustal involvement, or 2) the high-flux event resulted from remelting of underthrusted Outboard Darran Suite, together with more volumetrically significant supracrustal involvement.

We investigate possible relative contribution of supracrustal material during the arc flare-up through mass balance calculations assuming the following mean zircon-normalized d¹⁸O values: mantle (5.3 ‰), supracrustal 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 (hypothesis 1) yield values ranging from 0-20% supracrustal input during the flare-up. In the case of hypothesis 2, binary mixing of remelted Darran Suite with DCG requires 25-40% supracrustal input. Although both scenarios are possible, the lack of observed inherited Darran-age zircons and the homogeneity of zircon d¹⁸O values in the WFO suggests that the high-flux event resulted largely from mantle melting with 0-20% supracrustal input. These results contrast with those from North American batholiths where >50% of the arc mass is recycled crust.