2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 21
Presentation Time: 8:00 AM-12:00 PM


TRZASKUS, A.P.1, KING, E.M.1, VALLEY, J.W.2 and GILLETTE, R.O.1, (1)Dept. of Geography-Geology, Illinois State University, Normal, IL 61790, (2)Department of Geology and Geophysics, Univ of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI 53706, aptrzas@ilstu.edu

The St. Francois Mountains of Missouri represent the only major exposure of Mesoproterozoic (1.4-1.5 Ga) rocks in the mid-continent region. The rhyolitic, dacitic, and granitic rocks are interpreted as the remnants of multiple Proterozoic caldera complexes containing ash-flow tuffs, ring intrusions, and resurgent domes. The rocks have A-type chemical characteristics and all of the plutonic rocks suggest shallow emplacement under low PH2O.

Zircon δ18O values are well within the observed range of Proterozoic zircons, but significantly more variable than other Mesoproterozoic zircons of the mid-continent. The St. Francois Mountains δ18O(Zrc) values are 7.08±1.10‰ (n=5 rocks); elevated and more variable than other 1.4 Ga granitic samples from the Arbuckle Mountains of Oklahoma (6.11±0.12‰; n=7 rocks). There are no observed low δ18O(Zrc) values (<4.5‰) in the St. Francois Mountains as seen in other caldera complexes. In fact δ18O(Zrc) values rise over 2‰ from the oldest to the youngest samples through the development of the complex.

Quartz from the cataclysmic ash-flow tuff units, ring intrusions, and resurgent domes of the eastern Butler Hill caldera averages 9.05±1.84‰ (n=21 rocks). The younger Taum Sauk caldera units contain quartz that is not only higher but also more variable (ave=11.53±3.12‰; n=11). The cataclysmic ash-flow tuff units of the Taum Sauk caldera from Johnson Shut-Ins are the highest with 16.16±0.44‰ (n=3). The whole-rock values from Johnson Shut-Ins are equally elevated with an average value of 14.11±0.87‰ (n=6). The Johnson Shut-Ins units appear to have interacted with water at low temperature either during or shortly after eruption since the δ18O(Qtz) is nearly 5‰ higher than the resurgent dome intrusions that post-date the ash-flow tuffs. The resurgent intrusions also have near magmatic Δ(Qtz-Zrc). No correlative alteration event appears to have been experience by the volcanic units in the Butler Hill caldera and also no regional alteration event appears to have homogenized δ18O values of the units in either caldera. The presence of the high δ18O units could explain the increase of δ18O(Zrc) from the older units at 6.05‰ to the youngest units at 7.90‰. Stoping or assimilation of the high δ18O rocks into the magma chamber could increase the magmatic δ18O during the progression of magmatic pulses.