Arkansas

Geochronology of the Arkansas Alkaline Province, Southeastern United States

Eby, G. N., Vasconcelos, P.

The Arkansas alkaline province (AAP) consists of intrusive bodies ranging in lithology from carbonatite and lamproite through nepheline syenite. Apatite and titanite fission-track ages fall into two groups – 101 to 94 Ma and ~88 Ma. New ⁴⁰Ar/³⁹Ar ages and those previously reported in the literature define a third age group of ~106 Ma. Apatite and titanite fission-track ages are concordant indicating rapid cooling due to the emplacement of these intrusions at high levels during a time of regional uplift. There is a relationship between age and petrology: (1) lamproites are emplaced at ~106Ma, (2) carbonatites and associated silica-undersaturated rocks are emplaced between 101 and 94 Ma, and (3) large nepheline syenites bodies are emplaced at ~88 Ma. Chemical and isotopic data support the inference that the lamproites are derived from subcontinental lithosphere while the other sequences are derived from the asthenosphere. The ages for the AAP, and other conflicting information, does not support the hypothesis that the AAP was formed by the passage of the North American plate over the Bermuda hotspot. A more likely explanation is that the magmatism was related to extension and re-activation of faults associated with the Mississippi Graben.

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Two Distinct Sets of Magma Sources in Cretaceous Rocks from Magnet Cove, Prairie Creek, and Other Igneous Centers of the Arkansas Alkaline Province, USA

Duke, G. I., Carlson, R. W., Eby, G. N.

Two distinct sets of magma sources from the Arkansas alkaline province (~106-89 Ma) are revealed by Sr-Nd-Pb isotopic compositions of olivine lamproites vs. other alkalic rock types, including carbonatite, ijolite, lamprophyres, tephrite, malignite, jacupirangite, phonolite, trachyte, and latite. Isotopic compositions of diamond-bearing olivine lamproites from Prairie Creek and Dare Mine Knob point to Proterozoic lithosphere as an important source, and previous Re-Os isotopic data indicate derivation from subcontinental mantle lithosphere. Both sources were probably involved in lamproite generation. Magnet Cove carbonatites and other alkalic magmas were likely derived from an asthenospheric source.

Lamproite samples are isotopically quite different from other rock types in Sr-Nd-Pb isotopic space. Although three lamproite samples from Prairie Creek have a large range of SiO2 contents (40-60 wt %), initial values of εNd (-10 to -13), 206Pb/204Pb (16.61-16.81), 207Pb/204Pb (15.34-15.36), and 208Pb/204Pb (36.57-36.76) are low and similar. Only 87Sr/86Sr(i) displays a wide range in the Prairie Creek lamproites (0.70627-0.70829). A fourth lamproite from Dare Mine Knob has the most negative εNd(i) of -19. Lamproite isotope values show a significant crustal component and isotopically overlap subalkalic rhyolites from the Black Hills (SD), which assimilated Proterozoic crust.

Six samples of carbonatite, ijolite, and jacupirangite from Magnet Cove and Potash Sulphur Springs exhibit the most depleted Sr-Nd isotopic signatures of all samples. For these rock types, 87Sr/86Sr(i) is 0.70352 - 0.70396, and εNd(i) is +3.8 - +4.3. Eight other rock types have a narrow range of εNd(i) (+1.9 - +3.7), but a wide range of 87Sr/86Sr(i) (0.70424 - 0.70629). These 14 samples comprise a fairly tight cluster of Pb isotopic values: 206Pb/204Pb (18.22-19.23), 207Pb/204Pb (15.54-15.62), and 208Pb/204Pb (38.38-38.94), suggesting very little crustal assimilation. They are most similar to EM-2 (sub-group of OIB).

Published ages of crustal amphibolite xenoliths from the Prairie Creek lamproite are Proterozoic (~1.32- 1.47 Ga), in keeping with isotopic evidence for crustal assimilation, including Tdm = 1.3-1.7 Ga. Published ages of lamproite (~106 Ma) indicate that these magmas intruded first, whereas carbonatites and other alkalic magmas were later (~102 to ~89 Ma). Asthenospheric upwelling first melted lithospheric mantle and crust, producing lamproitic magmas; asthenospheric magmas followed as swelling of the lithosphere ensued.

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Geochronology, Geochemistry and Petrogenesis of the Arkansas Alkaline Province

Eby, G. N.

The intrusives of the Arkansas Alkaline Province (AAP) lie in a northeast trending belt along the northern edge of the Mississippi Embayment. There is a spatially-related variation from lamproites at the southwestern end through several carbonatite complexes, a lamprophyre dike swarm, and then at the northeastern end several large syenite bodies.

Fission-track titanite and apatite ages suggest two discrete periods of magmatic activity: one ca. 100 Ma which includes most of the intrusions and the second at ca. 88 Ma which is the time of emplacement of the syenite bodies. In general the fission-track ages are in agreement with radiometric ages obtained by other methods. Of note is that for all the intrusions, with the exception of Magnet Cove, titanite and apatite ages are indistinguishable indicating rapid cooling for these intrusions. At Magnet Cove an age differential of ca. 6 million years (titanite = 102 Ma and apatite = 96 Ma) suggests slower cooling and a greater depth of emplacement. At present there is no convincing evidence for a regular time progression that would indicate a hot-spot trace.

Existing Sr, Pb and Nd isotopic data indicate that the source of the various magmas was a LIL-depleted mantle, as seen for other alkaline provinces. There is a good correlation between Zr and Hf for all the intrusives and Zr/Hf = 44, typical of a LIL-depleted mantle source, for the province. Nb/Ta and Th/U ratios are highly variable indicating late- and post-magmatic redistribution.

Modelling based on phase equilibria constraints and trace element data suggest that the lamproites and mafic dikes of the various intrusions can be related through variable degrees of partial melting of a garnet lherzolite mantle. The syenites are seen to have a separate source and differentiation most likely occurred at depth with emplacement of the felsic residua at high levels. Within the syenite bodies differentiation was largely controlled by feldspar fractionation and accumulation. The existing data support the idea that the emplacement of the various intrusions of the AAP should be viewed as two separate events separated by approximately 12 million years.

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