DETRITAL ZIRCONS FROM THE MAURICE MOUNTAIN QUARTZITE AND BLACK LION CONGLOMERATE, PIONEER MOUNTAINS, SW MONTANA: THE SOUTHERN EDGE OF THE BELT BASIN IN MISSOULA GROUP TIME

Correlation and structural stacking of non-fossiliferous sandstone and conglomerate in the Pioneer Mountains, SW Montana have been uncertain due to lack of definitive ways to distinguish the age of the units. We report U-Pb ages (using LA-ICP-MS at the University of Arizona) from detrital zircons from the Maurice Mountain quartzite (Zen, 1988) and underlying Black Lion Formation from west of Grace Lake. The Maurice Mountain contains the same detrital zircon populations as the Missoula Group of the Mesoproterozoic Belt Supergroup. On the other hand the Black Lion contains locally derived Archean zircons derived from the Dillon Block to the south.

Specifically two fine-grained intervals in the Maurice Mountain unit, a fine- to conglomeratic quartzite, contain identical detrital zircon populations to the upper Missoula Group, in particular a large 1726 Ma population (about 50% of the zircons) with subordinate populations at 2496 Ma, 1836 Ma, 1785 Ma, and 1442 Ma (syn-Belt).

The Black Lion pebble conglomerate and quartz sandstone contains almost entirely Archean zircon grains, with populations at 2476 Ma (30% of grains), 2599 Ma, 2787, 3286, and 3485 Ma (all about 10% of grains). The youngest zircon grain is 1798 Ma. The 2400 to 2800 Ma populations resemble those in two quartzite clasts from the Missoula Group Bonner Formation near Wise River. The Black Lion populations differ from those in the lower Belt LaHood Formation that contains mainly grains >3200 Ma (Ross and Villeneuve, 2003).

Two possibilities exist for the stratigraphic affinity of the conglomeratic Black Lion. 1) The Black Lion is a Cambrian rock, and lies below a klippen of Maurice Mountain Formation as shown by Zen (1988). In this case, Cambrian uplift of the Dillon Block would be required. Because there is no evidence of a fault in the well-exposed section at Grace Lake, this interpretation is problematic. 2) The Black Lion represents a proximally derived coarse facies of the Missoula Group with mainly Archean zircons, stratigraphically overlain by fine-grained Maurice Mountain quartzite that contains the distally derived Missoula Group Paleoproterozoic zircon populations mixed with locally derived quartzite clasts. We favor the second option.