SHRIMP U/PB STUDY OF DETRITAL ZIRCONS FROM THE ADIRONDACKS AND ADJACENT GRENVILLE PROVINCE OF CANADA; A PRELIMINARY REPORT

Flinton Group quartzites (Elzevir terrane) contain detrital zircons ~1150 Ma, are overprinted by ca 1090-1030 Ma Ottawan orogenesis, and occur near ca 1090 Ma plutons but lack detrital zircons of this age. These obsewrvations indicate that the Flinton is <1150 Ma and probably >1090 Ma. Six detrital zircons from a quartzite in the St. Boniface Group (Mauricie, Quebec)yielded one age <1176 Ma, but more dating is required to establish this as a minimum age (maximum age >1090 Ma Ottawan overprint). A preliminary study of 16 zircons from the extensive, and isoclinally folded (Ottawan) Irving Pond quartzite (IPQ) of the southern Adirondacks yielded 1.7-1.36 Ga ages for 12 grains and (1190-1133) +/- (20-15) Ma for another four. It is possible that 3 of these are metamorphic, but an 1163 +/- 13 Ma age is from the center of an elongate, prismatic grain with concentric oscillatory zoning. Much more work is needed to verify a post-1163 Ma depositional age, but this result currently suggests deposition in the interval 1163-1109 Ma. Dating of cross cutting diabase will restrict deposition to a narrower interval.

These results suggest regional clastic deposition shortly after emplacement of ca 1155 Ma AMCG magmas that overlapped with the final stages of the 1207-1160 Ma Shawinigan orogeny. This magmatism is attributed to delamination of the overthickened orogen and ascent of hot new athenosphere to the base of the crust. Regional presence of the <1150, >1090 Ma quartzites (and accompanying sediments) discussed here suggests orogen rebound following delamiantion accompanied by extensional collapse and the development of local sedimentary basins. The narrower, and older, the depositional time interval, the more rapid uplift and unroofing of the orogen. These conclusions require further detrital zircon ages, and the results will be critical for tectonic analysis.

Presence of Archean zircons in IPQ indicates that the Adirondacks, and terranes to the north, were reciving Superior Province detritus by ca 1155 Ma, and must have been part of or proximal to Laurentia. In contrast, detrital zircons in >1207 Ma metapelites are <1.37 Ga indicating deposition outboard of Laurentia.