Geological Society of America

T140 The Dance of Arcs: Causes of Arc Migrations, Flare-Ups, and Magmatic Focusing II

Monday, 23 October 2017
1:30 PM - 5:30 PM
Skagit 4

DO MID-CRUSTAL CORDILLERAN BATHOLITHS RECORD ARC FLARE-UP TRIGGERING PROCESSES?

Paper number: T140-12
Program: Technical Sessions
Time: 4:20 PM – 4:35 PM

Authors

Joshua J. Schwartz Presenting Author
California State University, Northridge, Northridge, CA
Andrew J. Tulloch
Institute of Geol & Nuclear Sciences, Dunedin, N/A
S. Andico
California State University Northridge, Northridge, CA
Harold H. Stowell
University of Alabama, Tuscaloosa, AL
Keith A. Klepeis
University of Vermont, Burlington, VT
Kouki Kitajima
University of Wisconsin–Madison, Madison, WI
John Valley
University of Wisconsin-Madison, Madison, WI

Abstract

Triggering mechanisms responsible for Cordilleran arc flare-ups are hotly debated and there is little consensus about whether magmatic surges are driven by upper-plate processes (e.g., underthrusting of fertile continental crust) or mantle processes (e.g., slab tears/break-off). Radiogenic and stable isotopes in shallow- to mid-crustal batholithic rocks are commonly employed to provide insights into flare-up processes; however, it is unclear whether these rocks accurately record triggering processes, versus intra-crustal processing during transport through the crust and/or deep-crustal remelting and mixing.

We compare and contrast isotopic signatures from both mid- and lower crustal batholithic rocks in the Early Cretaceous portion of the 260-105 Ma Median Batholith, Fiordland, New Zealand to provide insights into this debate. We find that zircons from mafic to intermediate rocks in the lower crustal Western Fiordland Orthogneiss record a narrow isotopic range, with pluton-average zircon δ¹⁸O values ranging from 5.3 to 6.1‰, and εHf values ranging from +2.9 to +5.0. These zircons show no evidence for interaction with crust, and we interpret them as having equilibrated with an enriched, sub-arc, lithospheric mantle melt. By contrast, mid-crustal rocks have more complex isotopic systematics that reflect variable interaction with a hydrothermally altered source. Eastern Separation Point Suite zircons from the Titiroa and Takahe plutons give δ¹⁸O zircon values ranging from 3.7 to 3.9‰, and εHf values ranging from +7.2 to +7.6. Western Separation Point Suite zircons from the Puteketeke, Refrigerator Orthogneiss and West Arm Leucogranite show a strong, E-W isotopic gradient with δ¹⁸O values ranging from 4.7 to 5.9‰, and εHf values ranging from +6.0 to +4.4. Isotopic modeling indicates that mid-crustal zircons are well described by 0-30% mixing of average Largs terrane (hydrothermally altered) crust with an enriched mantle melt. We conclude that lower crustal zircons record a mantle-driven flare-up involving an enriched, sub-arc lithospheric mantle source. In contrast, mid-crustal zircons display significant intra-crustal remelting and mixing. The absence of this signal in lower crustal plutons indicates that this crustal source was not a key factor in igniting the Cretaceous arc flare-up.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305307

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