THE MISSOURI DEVONITE PARADOX

Unusual Precambrian dikes locally known as “devonite” occur on Mt. Devon and as widely dispersed float in SE Missouri. They display feldspar megacrysts ≤ 10 cm enclosed by olivine tholeiite with rounded olivine (now antigorite), Ti-augite, 0.4-1.6 mm laths of labradorite, and opaques arranged in intergranular or subophitic textures and modified by mild propylitic-sericitic alteration. The color-zoned megacrysts display ovoidal pink cores marked by sericitic alteration so intense that optical identification is precluded. Minor kaolinite, phlogopite, epidote, hematite, chlorite, anthophyllite and stilbite are present. The cores are enclosed by less-altered greenish rims of optically-continuous, twinned plagioclase tapering inward along cleavages and fractures. Analyses presented by Johannsen (1937) and Goldich and Muilenburg (1939) cannot be accepted for any single ternary or binary feldspar. These workers concluded that the megacrysts record large plagioclase phenocrysts modified by hydrothermal action. In contrast, we consider them to be xenocrysts composed of two binary feldspars, a K-rich core enclosed by a reaction rim of plagioclase (rapakivi). We contend that Missouri “devonite” is a hybrid rockformed by mingling of basaltic melt with partially or completely crystallized granite pegmatite prior to aqueous fluid influx. Our evidence is:

1. Two generations of plagioclase are improbable in intergranular/subophitic textures.

2. It is unlikely that potassic fluid would alter cores of large plagioclase phenocrysts leaving rim and host plagioclase in a less altered state.

3. Euhedral rim plagioclase suggests an approach to equilibrium with basaltic melt.

4. The megacrysts are veined and embayed by host basalt.

5. Alkali feldspar cores are implied by chemical data, sericitic alteration, Carlsbad twins, interfacial angles and exsolved perthite (now smectite) in {010} sections at 77◦ to {001}.