LOWER-PLATE CONTROLS ON MAGMATIC TEMPOS IN THE ZEALANDIA CORDILLERA

The episodic nature of magmatism in Cordilleran arc systems is commonly described as 'orogenic cyclicity' whereby arc flare-up events, or high-magma addition rate events (MAR), occur every 30–70 m.y. and are separated by magmatic lulls. In the North and South American Cordillera, orogenic cyclicity is observed in many arc segments and has been linked to upper plate deformational processes and/or arc migration events. Outside of the North and South American Cordillera, it is unclear whether orogenic cyclicity is a worldwide phenomenon, and whether similar upper-plate processes are responsible for triggering arc flare-up events.

Here, we evaluate the magmatic tempo and processes that led to arc flare-up events in the Zealandia Cordillera segment of the paleo-Pacific margin of southeast Gondwana. This ‘sister orogen’ to the American Cordillera preserves a nearly complete crustal section from ca. 5-65 km paleo-depth and was active from ca. 500-100 Ma. A compilation of over 350 new and existing zircon dates from the lower, middle and upper crust shows that magmatism in the Zealandia Cordillera was dominated by two high-MAR events in the Devonian (370-368 Ma) and in the Early Cretaceous (129-105 Ma). The intervening magmatic lull lasted 250 m.y. and was characterized by a prolonged magmatic quiescence characterized by irregularly spaced low-MAR events. This spacing between high-MAR events is 3 to 4 times longer than average magmatic cycles recognized in the Mesozoic American Cordillera (~60–70 m.y.) and 8 to 13 times longer than the Cenozoic American Cordillera (~20–30 m.y.). We observe no evidence for orogenic cyclicity like those that characterize the North and South American Cordillera, and the two high-MAR events have distinct chemistries (S-type for the Devonian, and I-type for the Cretaceous) that cannot be related by similar or cyclical processes. Like other Cordilleran arc systems, the Zealandia Cordillera high-MAR events were associated with upper-plate deformation; however, the magmatic flare-ups were driven by changes in the geometry of the lower plate (slab roll-back and slab break-off) rather than upper-plate dominated processes. Our study demonstrates that dynamic changes in the lower plate may be a primary control on magmatic tempos where orogenic cyclicity is absent or highly irregular.