THE DEVONIAN AVON ALKALINE PROVINCE, MISSOURI: CHARACTERIZATION OF SUBCONTINENTAL MANTLE SOURCE AND EVOLUTION FROM OLIVINE PHENOCRYSTS

We present new data on the crystallization age of, and composition of olivine phenocrysts within, an alnöite and olivine melilitite of the Avon Alkalic Igneous Province (AAIP) of Missouri. The AAIP is an ultramafic igneous province consisting of more than 80 known lithologically and texturally diverse intrusions, cropping out in northeastern flank of the St. Francois Mtn. Terrane. ⁴⁰Ar/³⁹Ar geochronology of biotite phenocrysts constrains emplacement to 386 +/- 1 Ma. Xenocrystic biotite from one sample yields 40Ar/39Ar age spectra characteristic of episodic loss, indicating crystallization at ca. 1.3 Ga followed by partial loss in the ultramafic magma at 386 Ma. Olivines within the alnöite are subhedral, variably serpentinized, and embayed. Olivines within the melilitite are euhedral, but extensively serpentinized. Disequilibrium textures observed in alnöite olivine are consistent with resorption of magmatic olivine as a result of decompression during crystallization. Euhedral olivine within the melilitite appear to have remained in equilibrium with melt, suggesting derivation of alnöite and melilitite from unique magmas. Major and trace elemental abundances of olivine from the alnöite were characterized with electron probe microanalysis. Olivines are Mg-rich (Fo_86.9-Fo_89.9), and exhibit systematic variation in trace element (e.g., Ni (1627 to 3580 ppm), Cr (97 to 1603 ppm), Co (149 to 259 ppm), Ti (11 to 267 ppm), Al (undetectable to 923 ppm), and P (undetectable to 433 ppm)) abundances with decreasing forsterite content consistent with fractional crystallization. All geothermometers yield a range in temperature, e.g., the Al in olivine (De Hoog et al., 2009) yield temperatures of 1087° to 1313° C at depths of 80 km to 180 km (modern-day midcontinental LAB). Olivine trace element discrimination diagrams indicate AAIP magmas were derived from mantle sources with an alkalic affinity, similar to other continental alkaline rocks and kimberlite. A mantle origin via partial melting of carbonated peridotite mantle is suggested due to the high Mg content, results of geothermometric modeling, and high Ca and Ti abundance within olivine phenocrysts. Melting of the mantle may have ben triggered by "Acadian" tectonic events.