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Book Chapter

Geochronology of Precambrian Rocks in the St. Francois Mountains, South eastern Missouri

By
M.E. Bickford
M.E. Bickford
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D.G. Mose
D.G. Mose
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Published:
January 01, 1975

The St. Francois Mountains are the principal exposure of an extensive terrane of Precambrian silicic volcanic and epizonal intrusive rocks that extends from central Wisconsin and northern Ohio at least to the Texas Panhandle. Most of the volcanic rocks exposed in the St. Francois Mountains are ash-flow tuff intruded by rocks of granitic composition that have petrographic features indicative of shallow crystallization.

U-Pb measurements on suites of cogenetic zircons from most of these rocks yield ages of about 1,500 m.y., but zircons from the Munger Granite Porphyry are apparently only 1,400 m.y. old. Rb-Sr whole-rock isochrons from the plutonic and volcanic rocks yield ages close to 1,300 m.y. Calculated Rb-Sr model ages for individual samples are inversely related to Rb/Sr and are directly related to Sr concentration. The data suggest that Rb-Sr ages have been lowered by Sr loss.

It is concluded that the U-Pb ages of the zircon suites are the best estimates of the time of crystallization of the rock bodies and that the Rb-Sr ages are related to an event in the subsequent history of the rocks during which Sr was lost. This event was most likely a period of extensive hydrothermal alteration, possibly the one that caused emplacement of iron ore bodies. The petrographic similarity of rocks in the larger, mostly buried terrane to those of the St. Francois Mountains suggests that Rb-Sr ages of these rocks may also be too low if the rocks experienced similar subsequent alteration. The rocks of the St. Francois Mountains are similar in age to numerous anorogenic plutons in Wisconsin, the Rocky Mountains, and elsewhere in the Southwest.

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