MAGMATIC VARIABILITY AND ALTERATION HISTORY OF THE ST. FRANCOIS MOUNTAINS, MISSOURI : OXYGEN ISOTOPE RATIOS OF ZIRCON AND QUARTZ

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.