Stable and transient isotopic trends in the crustal evolution of Zealandia Cordillera

We present >500 zircon delta O-18 and Lu-Hf isotope analyses on previously dated zircons to explore the interplay between spatial and temporal magmatic signals in Zealandia Cordillera. Our data cover similar to 8500 km(2) of middle and lower crust in the Median Batholith (Fiordland segment of Zealandia Cordillera) where Mesozoic arc magmatism along the paleo-Pacific margin of Gondwana was focused along an similar to 100 km wide, arc-parallel zone. Our data reveal three spatially distinct isotope domains that we term the eastern, central, and western isotope domains. These domains parallel the Mesozoic arc-axis, and their boundaries are defined by major crustal-scale faults that were reactivated as ductile shear zones during the Early Cretaceous. The western isotope domain has homogenous, mantle-like delta O-18 (Zrn) values of 5.8 +/- 0.3 parts per thousand (2 St.dev.) and initial epsilon(Hf) (Zrn) values of +4.2 +/- 1.0 (2 St.dev.). The eastern isotope domain is defined by isotopically low and homogenous delta O-18 (Zrn) values of 3.9 +/- 0.2 parts per thousand and initial epsilon(Hf) values of +7.8 +/- 0.6. The central isotope domain is characterized by transitional isotope values that display a strong E-W gradient with delta O-18 (Zrn) values rising from 4.6 to 5.9 parts per thousand and initial epsilon(Hf) values decreasing from +5.5 to +3.7. We find that the isotope architecture of the Median Batholith was in place before the initiation of Mesozoic arc magmatism and pre-dates Early Cretaceous contractional deformation and transpression. Our data show that Mesozoic pluton chemistry was controlled in part by long-lived, spatially distinct isotope domains that extend from the crust through to the upper mantle. Isotope differences between these domains are the result of the crustal architecture (an underthrusted low-delta O-18 source terrane) and a transient event beginning at ca. 129 Ma that primarily involved a depleted-mantle component contaminated by recycled trench sediments (10-20%). When data showing the temporal and spatial patterns of magmatism are integrated, we observe a pattern of decreasing crustal recycling of the low-delta O-18 source over time, which ultimately culminated in a mantle controlled flare-up. Our data demonstrate that spatial and temporal signals are intimately linked, and when evaluated together they provide important insights into the crustal architecture and the role of both stable and transient arc magmatic trends in Cordilleran batholiths.

Automated summary

The study reveals three spatially distinct isotope domains in the Zealandia Cordillera, each corresponding to the eastern, central, and western regions along with major crustal-scale faults, suggesting a pre-existing isotope architecture before Mesozoic arc magmatism in the Early Cretaceous. The isotopic differences in the Median Batholith are controlled by long-lived domains extending from the crust through to the upper mantle, with varying delta O-18 and epsilon(Hf) values. The interplay between spatial and temporal signals indicates a decreasing crustal recycling over time leading to a mantle controlled flare-up in the Mesozoic magmatism.