Xenolith mineralogy and geology of the Prairie Creek lamproite province, Arkansas.

摘要

Three new Arkansas lamproites were delineated and evaluated during the 1980's. The intrusions were emplaced during the Middle Cretaceous Period based on stratigraphic relationships and published isotopic ages. Geological relationships record a dominant crater facies lamproite with minimal vent erosion. A 260 tonne sample taken from the two larger lamproite vents recovered 5 macro diamonds (>0.5mm) yielding a sub-economic diamond grade of ∼0.04 carats per 100 tonnes. A diamond evaluation program undertaken at the Prairie Creek vent by the Arkansas State Parks Commission in the 1990's resulted in mapping of four major rock types: epiclastic rocks, olivine lamproite, phlogopite-rich tuff and olivine-rich tuff. Significant diamond contents were found only within the phlogopite-rich tuff (∼0.11 carat/100 t) and olivine-rich tuff (∼1.1 carat/100 t). Stratigraphic relationships indicate that the diamondiferous tuffs have undergone 50 meters of erosion. Extrapolation of the surface rock units and their diamond contents to the pre-erosion surface suggests that ∼93,000 carats of diamonds were liberated and then concentrated as a natural surface enrichment.;Mantle xenoliths recovered from the Black Lick and Twin Knobs lamproite vents were analyzed for major element compositions. Their data were used to calculate a pressure-temperature array that record maximum pressures of ∼5 GPa and maximum temperatures of ∼1000°C for the xenolith source region. Comparisons between calculated pressures and olivine compositions indicate relatively shallow fertile mantle overlying more depleted mantle lithosphere. The two layers of mantle lithosphere may represent different ages based on their olivine composition. Crustal xenoliths include near-surface sedimentary rocks and abundant amphibolite with granitoids and rare meta-sedimentary and meta-volcanic rocks. K-Ar dates of 1.48--1.31 Ga were obtained from four amphibolite xenoliths. The cooling ages confirm that continental crust of ∼1.42 Ga in age extends beneath southwestern Arkansas and that thermal effects of the younger Grenville and Ouachita orogenies were insufficient to reset the amphibole K-Ar systems. Xenolith data and published results are used to test two models for development of the Ouachita system. The first model proposes that the Ouachita trough is part of a mid-continent failed rift; the second model suggests the Ouachita System was formed at the rifted oceanic margin of the continental craton. The xenolith data support the intra-craton rift model over the oceanic-margin model.