HR: 0800h
AN: T41D-1253    [Abstracts]
TI: New SHRIMP-Zircon Data Provide Evidence for a Complex Polyphase Magmato-Metamorphic History of the Western Bohemian Massif
AU: * Wiegand, B A
EM: bwiegand@pangea.stanford.edu
AF: Dept of Geological and Environmental Sciences, Stanford University 367 Panama Mall , Stanford University, CA 94305
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: Dept of Geological and Environmental Sciences, Stanford University 367 Panama Mall , Stanford University, CA 94305
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U.S Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AB: Zircons from three orthogneiss complexes and a Sill-granitoid from the Saxothuringian-Moldanubian transition zone (Western Bohemian Massif) have been studied using SHRIMP-RG. The results suggest a complex polyphase magmatic and metamorphic history for the rocks in this area. The latest metamorphic overprint occurred during Variscan LP/HT regional metamorphism between 330-320 Ma. Conventional U-Pb zircon dating of several orthogneiss and pegmatite complexes of the Saxothuringian, the Moldanubian and the nappe complexes (ZEV, ZTD, MM) produced ages between 460 and 530 Ma which were interpreted as emplacement and crystallization ages for these rocks. Upper concordia intercept ages between 2000 Ma and 2700 Ma were interpreted to be ages of source materials. Conventional U/Pb analyses of zircon from the polyphase overprinted Saxothuringian-Moldanubian transition zone often result in extremely discordant ages influenced by lead loss, high common lead contents, and uranium gain (Wiegand 1997). Hence, conventional U-Pb age determinations should be used with caution. SHRIMP analyses provide more insight into the age pattern in zircons from the Saxothuringian-Moldanubian transition zone. The results show a widespread distribution of ages between 200 Ma and 3200 Ma with main peaks at approximately 470 Ma and 570 Ma. Both ages reflect magmatic stages and are found together in the investigated zircon separates. For the Wunsiedler Gneis and the Epigneis of Wernersreuth (Saxothuringian) the youngest age peak is 490 Ma and reflects the last magmatic stage and emplacement of these rocks. Two major older age peaks reflect an earlier magmatic stage at approximately 570 Ma and an inherited magmatic event at about 610 Ma. An additional small amount of older inherited zircons suggest previous magmatic events at approximately 1000 Ma and 1950 Ma. The latest metamorphic overprint occurred during Variscan regional metamorphism at about 320 Ma ago. The age pattern of the Tachov Orthogneiss and the Sill-granitoid (Moldanubian) is far more complex. Magmatic events are comparable to the Saxothuringian orthogneisses at approximately 460 Ma and 570 Ma. In addition, a widespread older age pattern of approximately 650 Ma, 800 - 1200 Ma, 2700 Ma, and 3200 Ma reflects inherited ages of earlier crustal events. The Sill-granitoid intruded Moldanubian rocks during the Variscan orogeny which is evident from a strongly zonated zircon population formed at about 320 Ma ago and monazites of the same age. The last magmatic stage of the Tachov Orthogneiss appears to be at about 460 Ma with later metamorphic overprint during Variscan LP/HT metamorphism which was followed by late stage hydrothermal activity. Lead loss during metamorphism is believed to be responsible for apparent ages between 330 and 200 Ma. B. Wiegand (1997) Geotektonische Forschungen 88, Schweizerbart, Stuttgart, Germany.
DE: 9800 GENERAL OR MISCELLANEOUS
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting