HR: 0800h
AN: GP11D-0877    [Abstracts]
TI: Remagnetization of Lower Carboniferous Carbonates, Northeastern Ireland: Preliminary Paleomagnetic Results
AU: * Pannalal, S J
EM: pannala@uwindsor.ca
AF: Dept. of Earth Sciences, University of Windsor, Windsor, ON N9B 3P4 Canada
AU: Symons, D T
EM: dsymons@uwindsor.ca
AF: Dept. of Earth Sciences, University of Windsor, Windsor, ON N9B 3P4 Canada
AU: Stanley, G
EM: gerry.stanley@gsi.ie
AF: Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin, 4 Ireland
AU: Sangster, D F
EM: dsangster@sympatico.ca
AF: Consultant, 2335 Russvern Drive, North Gower, ON K0A 2T0 Canada
AB: Conodont color alteration index (CAI) values provide indirect paleotemperature estimates for rocks that have been subjected to a thermal event either by burial metamorphism or by hot hydrothermal fluids. Extensive CAI studies have been conducted on the Lower Carboniferous carbonates of Ireland that are host to the major sedimentary lead and zinc deposits in the Irish ore field. The CAI values show that these rocks have been affected by a regional thermal event with a decreasing trend from south (CAI=7.0) to north Ireland (CAI=1). In addition, the relative timing of the regional metamorphic event and ore mineralization in the Irish ore field, critical to the much debated genetic theories for ore genesis, has remained elusive. A paleomagnetic study, that includes thermal and alternating-field demagnetization and isothermal remanence procedures, has been conducted on 235 specimens representing 18 sites from Lower Carboniferous carbonates (CAI $<$ 3.0) of northeastern Ireland to date the metamorphism. Preliminary results indicate that the characteristic remanent magnetization (ChRM) in 144 specimens from 12 sites (sites 7-18) is carried by single- to pseudo-single domain pyrrhotite and/or magnetite with a mean direction (component A) at D=192.8$\deg$, I=3.0$\deg$ ($\alpha$$_{95}$=4.5$\deg$). In contrast, 67 specimens from 6 sites (sites 1-6) with fine to coarse hematite as the chief magnetic remanence carrier have a mean ChRM direction (component B) at D=17.8$\deg$, I=32.9$\deg$ ($\alpha$$_{95}$=9.4$\deg$). Both the A and the B component ChRM directions were corrected for the tilting of the beds. The A component direction is similar to the ChRM direction isolated for the Lower Carboniferous host carbonates of the Navan lead and zinc deposit in the Irish Midlands. This suggests that a large-scale hydrothermal fluid event may be responsible for a regional remagnetization of the carbonates. Conversely, the B component direction is significantly different and younger in age than the A component and may indicate a secondary magnetization, likely the result of weathering of the carbonates by oxidizing fluids. In addition, this regional hydrothermal event may also be related to the observed CAI values in Lower Carboniferous carbonates of northeastern Ireland.
DE: 9335 Europe
DE: 9614 Paleozoic
DE: 3665 Mineral occurrences and deposits
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1533 Remagnetization
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting