CONSTRUCTION OF THE MEDIAN BATHOLITH, FIORDLAND, NEW ZEALAND

We investigated the temporal record of magmatism in the Fiordland sector of the Median Batholith in New Zealand with the goal of evaluating linkages between magmatism and deformation in continental arcs. Zircon chronology reveals that Mesozoic construction involved at least two high magma addition rate (MAR) events: a surge of low Sr/Y plutonism in the Darran Suite from 140 to 136 Ma, and a terminal surge of high Sr/Y magmatism in the Separation Point Suite from 128 to 114 Ma shortly before the extensional orogenic collapse of Zealandia Cordillera began at 108-106 Ma. The latter high MAR event occurred at all depths within the crust, but was concentrated in the lower crust where ~50% of the arc root consists of Cretaceous plutonic rocks. Dates from lower crustal plutonic rocks (named the Western Fiordland Orthogneiss) primarily range from 118-115 Ma and signify a major flux of mafic to intermediate magmatism during which nearly 70% of the arc root was emplaced. The spatial distribution of dates reveals magmatic focusing towards the center of the Western Fiordland Orthogneiss during peak magmatic activity. This event marks the termination of a prolonged (>100 Ma) period of magmatism in the Median Batholith that lasted from ca. 225 to 114 Ma in Fiordland.

Like other Cordilleran orogenic systems, we observe cyclical variations in high MAR events with a ca. 20 Ma periodicity, though causes for magmatic surges remain unclear. In the case of the terminal Cretaceous surge, the high MAR event is associated with a) transpression and regional thrusting from ca. 130-105 Ma, b) crustal thickening and possibly loading from 128-116 Ma, and c) continentward migration of arc magmatism. Zircon oxygen isotope data from the Western Fiordland Orthogneiss range from 5.30 to 6.05 and yield an average value of 5.76 ± 0.23 ‰ (1SD; n=126), consistent with a mantle source. Zircon Hf isotopes from the same samples yield an average initial eHf value of +4.2 ± 0.5 (1SD; n =354), and whole rock and apatite 87Sr/86Sr values give an average of 0.7044 ± 0.0005 (1SD; n =16). These observations indicate that the terminal Zealandia surge was primarily triggered by mantle processes (e.g., slab break off) with only limited contributions from upper plate materials. This high flux event was a precursor (and may be linked) to extensional orogenic collapse of Zealandia Cordillera.