Field Forum
Report
Structural Controls on Magma Transport and Vertical Coupling in the Continental Lithosphere
April 26-May 6, 2003
Fiordland, New Zealand
Leaders:
Keith Klepeis, University of Vermont
Geoffrey Clarke, University of Sydney
Tracy Rushmer, University of Vermont
Andrew Tulloch, Institute of Geological and Nuclear Sciences
This Field Forum was held in Fiordland National Park on the South Island of New Zealand. The Fiordland setting allowed us to examine processes that affect crustal evolution at unusually deep levels of an ancient arc (25-50 km paleodepth). Our goal was to use these exposures to test our understanding of arc structure and magma generation and migration through the lower crust. The trip began with a two-day workshop where participants from seven countries met to discuss relevant work. Fields of expertise included igneous, metamorphic, and experimental petrology; geochemistry; structural geology; geodynamics; and volcanology. Workshop discussions were mixed with helicopter trips to nearby exposures. Following the workshop, we accessed shoreline exposures from aboard the Milford Wanderer for five days. The following topics focused our discussions.
Milford Wanderer edges close to an outcrop along the shores of Caswell Sound, Fiordland National Park. Photo by A. Tullock. Click on image for larger photo. |
Melt segregation and transfer mechanisms in the lower crust
The Fiordland exposures provided evidence that diking and melt-enhanced fracturing aided melt transfer through the lower crust. Structural elements in ductile shear zones also were discussed as pathways for melt transport. Geoff Clarke, Nathan Daczko, Peter Robinson, and Harold Stowell led discussions on the effects of fluid and melt migration, including crustal dehydration. Tracy Rushmer and Alberto Patino-Douce emphasized the role of mineral reactions in controlling melt volumes and segregation mechanisms. Simon Harley led discussions on how hot fluids may affect zircon chemistry and can influence interpretations of crystallization and metamorphic ages.
Variations in magma composition
Andy Tulloch presented evidence for two distinctive margin-parallel plutonic suites. Diorite and tonalite-granodiorite plutons displaying high Sr/Y ratios developed inboard of slightly older gabbro-diorite-granite plutons with lower Sr/Y ratios. Similar paired suites occur in other arcs and a HiSY (high Sr/Y), LoSY (low Sr/Y) terminology was introduced in preference to terms such as adakite, TTG (tonalite-trondhjemite-granodiorite). Ian Turnbull and Andrew Allibone pointed out that the lower crust is very heterogeneous compositionally (".a mixed bag containing a multitude of sins!"), and Robert Wiebe showed how the magmas interacted. Weecha Crawford and Julie Vry emphasized the effects of compositional variability on the mineral reactions controlling melt production. Jon Davidson and Michael Dungan explored what volcanoes tell us about magma generation and degree of mixing in the deep crust. We concluded that Fiordland may be an example where crust-mantle mixing occurred beneath a deep mafic intrusion and the rapid ascent of hybrid magmas inhibited shallow crustal contamination.
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Variations in structural style with crustal depth
Bob Miller, Jinny Sisson, and Cees van Staal led discussions on arc structure. Keith Klepeis showed that steep shear zones in the lower crust cut vertically upward to merge with a mid-crustal fold-thrust belt. Arthur Goldstein, Jim Dunlap, Brad Hacker, and Jeff Lee discussed how displacements were relayed vertically through the arc. Discussions on distinguishing extensional and contractional structures, led by George Gibson, and the Cenozoic reactivation of Cretaceous structures, led by Rupert Sutherland, also were important to the interpretations.
The Fiordland setting is stimulating new ideas about magma genesis and how deep-level processes relate to those in the shallow crust. The Field Forum confirmed the value of combining diverse fields of expertise to study crustal evolution. Discussions will continue at the American Geophysical Union December meeting in special session V09 ("Birth, growth and death of magmatic arcs: Comparisons among arcs in different settings") cosponsored by Volcanology, Petrology and Geochemistry and Tectonophysics.
Acknowledgments
We thank the GSA for sponsoring the Field Forum. Edna Collis and Gabriela Mora-Klepeis provided valuable assistance. Grants from the GSA Pretorius Fund and the National Science Foundation helped students attend the meeting. The Institute of Geological and Nuclear Sciences (New Zealand) provided logisitical support. We thank the Department of Land Conservation, Te Anau, for permission to visit park localities.