HR: 17:45h
AN: V54B-08 INVITED     [Abstracts]
TI: Unsolved Ultrahigh-Pressure Mysteries
AU: * Hacker, B R
EM: hacker@geol.ucsb.edu
AF: Geology, UC, Santa Barbara, CA 93117
AB: The abundance and scale of Phanerozoic UHP terranes imply that the subduction and exhumation of continental crust has had a major effect on the evolution of Earth. The relative youth of this field of inquiry, however, means that there are many fundamental questions about UHP tectonics that have not been definitively addressed: 1. Continental crust can be subducted in a variety of tectonic settings. Many of the best-known UHP terranes appear to have developed in the absence of a magmatic arc in the hanging wall. Is this a requirement for the formation or exhumation of UHP terranes or can UHP rocks form and be exhumed in all settings? 2. Contrary to expectations, the bulk gneisses in UHP terranes do not transform completely to UHP mineral assemblages. To what degree is this true and how does this affect the buoyancy evolution of the UHP slab? 3. Suggestions that the gneisses in UHP terranes are weakly deformed contrast strongly with the idea of widespread lower crustal flow. Why this paradox? 4. Are the multiple UHP events known from the best-studied orogens the result of repeated subduction of the same tectonostratigraphic unit or separate metamorphic events in different units? 5. What is the ultimate depth to which continental material is subducted and exhumed? To what extent has assimilation of continental rock into the mantle affected the chemical evolution of Earth? Can continental subduction produce (ultra)potassic magmatism? 6. Local body forces are insufficient to exhume UHP rocks in continental settings. Under what circumstances and with what frequency are UHP terranes exhumed to the surface as opposed to being arrested at deeper levels within the lithosphere and never exhumed? Has arrest of exhuming UHP terranes led to significant reworking of the continental crust? This presentation will offer specific means of resolving these questions through investigations that target specific UHP terranes and build on the general data that are available.
DE: 3654 Ultra-high pressure metamorphism
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005