LOWER-PLATE CONTROLS ON MAGMATIC TEMPOS IN THE ZEALANDIA CORDILLERA
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.