HR: 13:45h
AN: V43E-01 INVITED    [Abstracts]
TI: Ultrahigh Pressure Metamorphism (UHPM): Quo Vadimus?
AU: * Brown, M
EM: mbrown@umd.edu
AF: Univ. of Maryland, Lab. for Crustal Petrology, Dept. of Geology, College Park, MD 20742, United States
AB: UHPM is the petrologic record (min. P-T equivalent to Coe stability) of transport of continental lithosphere to asthenospheric depth, and return to crustal depth and incorporation into continents. The record of UHPM is scale- independent, but the issue of unit size (boudin/slice/terrane) and whether UHPM is recorded are important. Although the requirement for external hydration may limit equilibration in protoliths, cofacial compatible, indicative and diagnostic assemblages and lack of confidence in thermobarometry obfuscate the record of UHPM. Polymetamorphism and/or overprinting by multiple UHPM events may be possible through successive Wilson cycles and may be cryptic to add complexity. Our ability to image and study inclusion assemblages in zircons has been a major breakthrough, but are diamonds anybody's best friend? Is the max. depth from which continental crust may be retrieved equivalent to P = 10GPa or >10GPa? Will evidence remain to infer such depth? Commonly we tie dates to P-T, which yields rates that constrain mechanisms; advances in numerical modeling will enable progress in testing models of exhumation. UHPM is registered in the rock record since the Cryogenian-Ediacaran; it is inferred to record deep subduction. UHP rocks decorate sutures in Phanerozoic subduction-to-collision orogens. During the Mesoarchean-Tonian, subduction-to-collision orogenesis was marked by eclogite - high-pressure granulite metamorphism (E-HPGM). The implied change in geodynamics was a legacy of Rodinia, and related to the birth of the Pacific during the formation of Gondwana. Hoffman-type supercontinent breakup (turns inside out by subduction of complementary superocean) was the process by which segments of Rodinia were reassembled to form Gondwana by suturing of Braziliano - Pan-African belts, leaving orphaned Laurasian segments to combine with each other and Gondwana to form Pangea. In contrast, Wilson cycles (continental lithosphere rearranged by formation- destruction of internal oceans) dominated the Phanerozoic, when the continental lithosphere was restricted to one hemisphere. The formation of Laurasia, its amalgamation with Gondwana to form Pangea, and the fragmentation of Pangea involved successive major orogenic systems--the Appalachian/Caledonian-Variscide- Altaid and the Cimmerian-Himalayan-Alpine. The Iapetus, Rhenohercynian, Rheic, Paleo-Tethys and Neo-Tethys Oceans were closed by consumption of internally-generated ocean lithosphere by ribbon-continent terrane export from the Gondwanan side, and, for the Variscides, continent collision. Limited subduction of short-lived (warm) lithosphere before choking subduction by arc or ribbon-continent terrane collision may have limited transport of water to the mantle wedge, and suppressed development of small-scale convection, arc magmatism and backarc formation. In the complementary ocean hemisphere, circumferential subduction created accretionary orogens, commonly with paired metamorphic belts; whether UHPM is common in these orogens remains important to resolve. Why UHPM generally is associated with subduction of short-lived lithosphere is unclear? Large-scale mantle tomography for S-wave velocity structure of lower mantle supports a simple pattern of convection for modern Earth. High-velocity anomalies (colder) match Phanerozoic subduction zones and are interpreted as slabs. Low-velocity structures (LVS) display large positive geoid anomalies, shallow seafloor and elevated topography, and extensive hot spot volcanism, features indicating warmer mantle consistent with high- bulk-modulus domes composed of pyroxenite, linking them to subducted MORB. The breakup of Pangea was a legacy of Gondwana amalgamation (African LVS; a LVS is likely to develop under Asia in the future as a legacy of the formation of Asia.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting