HR: 1330h
AN: V32C-1026    [PDF]
TI: Young monazite ages timing extension and contraction: Problems and interpretations
AU: * Catlos, E J
EM: catlos@okstate.edu
AF: Oklahoma State University, School of Geology, 105NRC, Stillwater, OK 74078 United States
AU: Harrison, T
EM: mark.harrison@anu.edu.au
AF: The Australian National University, Research School of Earth Sciences, Canberra, ACT 0200 Australia
AU: Cemen, I
EM: icemen@okstate.edu
AF: Oklahoma State University, School of Geology, 105NRC, Stillwater, OK 74078 United States
AB: Monazite, a rare-earth and radiogenic phosphate mineral, is frequently used to time metamorphic and igneous events. The potential exists to date monazite in rock thin section using an ion microprobe; a method that preserves key textural features and facilitates our interpretation of the age. However, the application of monazite geochronology is hindered by a lack of understanding of the pressure-temperature (P-T) conditions at which the mineral forms in metamorphic rocks. In the Himalayas, monazites collected from the footwall of the Main Central Thrust (MCT), a crustal-scale shear zone long thought to be inactive since the Miocene, yield 1.0$\pm$0.5 Ma and 0.8$\pm$0.2 Ma Th-Pb ion microprobe ages. If the ages time hydrothermal monazite growth due to slip within the zone at a speculated temperature of $\sim$400$\deg$C, the MCT accommodates 40-54$%$ of the total convergence rate between the Indian and Eurasian plates and has faster shortening rates ($>$16 mm/yr) than those estimated for the seismically active Himalayan Main Frontal Thrust. In western Turkey, a monazite grain collected from the surface of the Kuzey Detachment, a low angle normal fault that bounds the Central Menderes Metamorphic Core Complex to the north, is 4.5$\pm$1.0 Ma. The grain is in reaction with allanite, and its age is within 1$\sigma$ of the apatite fission track result of 3.7$\pm$0.6 Ma from a rock collected along the same detachment. This result raises questions of the P-T conditions and paths required for the allanite to monazite reaction. The presence of these young monazite grains in two type localities of contraction and extension is significant, producing rapid exhumation and slip rates, and elevating the structures they were collected from to major roles. The rates they produce, however, have large uncertainties because of the currently speculative P-T conditions of monazite formation.
DE: 1035 Geochronology
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3660 Metamorphic petrology
DE: 8110 Continental tectonics--general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting