Paper No. 7
Presentation Time: 9:30 AM
A COMPARISON OF CHIME AND ID-TIMS U-PB MONAZITE AGES FROM THE ROCKY MOUNTAIN TRENCH NEAR GOLDEN, BRITISH COLUMBIA, CANADA AND THE IMPORTANCE OF CHIME ANALYSES
For this study the CHIME (chemical dating using the electron microprobe) technique provided valuable assistance in separating out samples that contained detrital cores in monazites. This is absolutely vital for studies pertaining to the timing of regional metamorphism. The current study area is located along the Rocky Mountain Trench (RMT), British Columbia, Canada, from lat. 52°ã 05¢N to lat. 51°ã 45¢N. In the study area, the Purcell Thrust, a regional scale out-of-sequence thrust with vertical throw between 7 and 10 km, lies within the RMT and is offset by the Trench Normal Faults. CHIME analyses were used in the study area to distinguish between samples with Neoproterozoic to Cambrian detrital monazites, regional metamorphic monazites (135 to 165 Ma hornblende 40Ar-39Ar ages) and monazites that grew during movement along the Purcell Thrust (116 [north] through to <85 Ma [south]). Five samples (six monazite populations) without detrital cores were chosen for ID-TIMS (thermal ionization mass spectrometer) analyses, including four samples with apparent regional metamorphic ages and two samples with apparent ages related to movement along the Purcell Thrust. Unfortunately when separated, the sample with two monazite populations (50 m into the hanging wall,101 +/- 15 Ma and 147 +/- 28 Ma), failed to reveal any monazites. This demonstrates a major strength of CHIME analyses, being able to analyze monazites of all sizes in-situ. During the separation process for ID-TIMS analyses, only grains with one dimension greater than »50 um are recoverable. The electron microprobe is capable of analyzing grains as small as 5 um. All of the ID-TIMS ages were between 23 and 54 Ma younger than the corresponding CHIME ages. One possible reason for this discrepancy is that the Pb and U contents in these monazites are very close to the microprobe detection limits. These ID-TIMS ages of from 61.2 +/- 0.5 Ma to 116.8 +/- 0.2 Ma are also close to the proposed lower limit for the CHIME technique. Also, each fraction analyzed by the ID-TIMS technique had common Pb ranging from 392 down to 1.31 pgm. The common Pb content was not consistent from fraction to fraction or from sample to sample. These variations are impossible to measure with the microprobe due to the microprobe being unable to differentiate between the different Pb and U isotopes.