THE GRANITE RHYOLITE PROVINCE OF NORTH AMERICA

The Granite Rhyolite Province (GRP) of the North American midcontinent is part of an extensive Mesoproterozoic felsic igneous belt which is located along the margin of Laurentia outboard of the Archean craton. This felsic belt extends from Finland to the western US and most likely Australia and the Antarctic. Not only is this unit geographically extensive it covers a large time period from 1.5 to 1.3 Ga with at least three distinct episodes at 1.47, 1.37, and 1.33 Ga. In general the GRP becomes younger to the south southwest, from the Eastern GRP to the Southern GRP. In the St Francois Mountains of Missouri and the Arbuckle Mountains of Oklahoma, two magmatic episodes are separated by approximately 100 my; however, the isotopic evidence suggests no change in crustal source rocks. The crustal units also become younger to the south southwest of the midcontinent.

In the midcontinent, researchers rely on the few surface outcrops and over 2000 well penetrations. A map distinguishing extrusive and intrusive samples from the well penetrations and surface exposures in the midcontinent reveals six rhyolitic centers which occur as topographic highs on the Precambrian basement. One of these centers is exposed, the St Francois Mountains, and another is defined by a set of closely spaced wells in the Texas panhandle.

Samples from the GRP are generally undeformed and consists of metaluminous to peraluminous rhyolite – both aphanitic and with phenocrysts of quartz and/or Kspar, trachyte, epizonal granite, quartz syenite and monzonite. Gabbro can locally be shown to be contemporaneous with the felsic units. Geochemically the rhyolites and granites have high to ultrahigh potassium, some are reducing with high Fe/Mg values and modal ilmenite, whereas others are oxidizing, with modal magnetite.

The location of the GRP, its time-transgressive nature over >100 m.y. time interval, and the predominance of highly evolved magmas is unlike modern arcs, rifts, or hot spots. Although trace element compositions which are interpreted to indicate “within-plate” characteristics, a definitive tectonic setting for the GRP remains to be determined.