Melt Extraction From the Lowermost Crust: An Experimental Test of a Natural Setting

University Of Vermont & State Agricultural College, Burlington VT

Investigators

Abstract

Rushmer EAR-0106241 Experimental studies on melt generation and extraction in crustal rock types are now providing testable models with which to more fully examine natural settings. The problem is that few field sites have the exposure necessary to study potential mechanisms of melt segregation via fracture and subsequent transport mechanisms. The Early Cretaceous granulite belt of Fiordland, New Zealand, however, is an exception. It is a deeply eroded continuous belt which exposes all levels of the crust, truly lowermost crustal rocks, in addition to middle and upper crust. Field observations of exposed lowermost crust have documented the presence of extensive leucosome-lined fracture networks which appear to have also been used for melt transport. Testing this scenario experimentally will provide an excellent opportunity to integrate these field observations with experimental data. We are therefore conducting an experimental investigation involving phase equilibria, static rock core and deformation studies on the source dioritic rocks to accomplish the following tasks: 1) test the hypothesis that partial melting in this hydrous-phase rich bulk composition produced the observed fractures lined with leucosome material; 2) determine melt compositions and melt-water contents produced by partial melting of diorite under granulite facies conditions to be used as tracers for melt transport; 3) determine strength behavior of the diorite during partial melting at high pressure; and 4) use the above experimental results to constrain petrologic and structural models of the evolution of the lowermost crust as exposed in the Fiordland granulite belt. Experimental determination of partial melt compositions and possible segregation mechanisms will be combined with field relationships, geochemical and petrologic studies to constrain sources and transport paths for partial melts observed in these locales in the Fiordland lower crustal section. The results of these combined studies have significant potential impact for furthering our understanding of melt-related processes in the lowermost crust and the role of melt during orogenic evolution. This three-year project provides direct support for three Masters students. Three undergraduates will participate in the project in the summers as part of their senior theses.