Geochronology of the Arkansas Alkaline Province, Southeastern United States Author(s): G. Nelson Eby and Paulo Vasconcelos Source: The Journal of Geology, Vol. 117, No. 6 (November 2009), pp. 615-626 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/10.1086/605779 . Accessed: 21/10/2015 21:10 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. http://www.jstor.org This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:10:48 PM All use subject to JSTOR Terms and Conditions Geochronology of the Arkansas Alkaline Province, Southeastern United States G. Nelson Eby and Paulo Vasconcelos1 Department of Environmental, Earth, and Atmospheric Sciences, University of Massachusetts, Lowell, Massachusetts 01854, U.S.A. (e-mail: [email protected]) ABSTRACT 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—101–94 Ma and ∼88 Ma. New 40Ar/39Ar ages and those 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 ∼106 Ma, (2) carbonatites and associated silica-undersaturated rocks are emplaced between 101 and 94 Ma, and (3) large nepheline syenite 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 do 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 reactivation of faults associated with the Mississippi graben. Online enhancement: appendix table. Introduction Cretaceous alkaline rocks are widespread through- posures of alkaline rocks are found in the Arkansas out the northern Gulf of Mexico basin, southeast- alkaline province (AAP), which consists of seven ern United States, but are only sparsely exposed at intrusions or intrusive complexes that lie along a the surface (Byerly 1991). Subsurface exposures of northeast–southwest trend that falls on the exten- alkaline rocks have been encountered in drill holes sion of the Mississippi Valley graben (fig. 1). In this in southeast Oklahoma, south-central Texas, article we report new apatite and titanite fission- northeast Louisiana, southeast Arkansas, and west- track and 40Ar/39Ar mica and whole-rock ages for central Mississippi. The four major groups of sub- the major intrusive bodies (with the exception of surface volcanic and hypabyssal rocks are tra- Prairie Creek) and a number of dikes from the AAP. chytes, phonolites, alkaline basalts, and lam- prophyres (Kidwell 1951). Baksi (1997) reports 40Ar/ Geology 0.4 ע 39Ar ages for these rocks ranging from 67.2 Ma. Volcanoclastic sediments of the 1.3 ע to80.7 Woodbine Formation in southwest Arkansas pri- The various intrusions of the AAP were emplaced marily contain phonolite and trachyte cobbles with into the folded and faulted lower to middle Paleo- lesser trachybasalt cobbles. Whole-rock 40Ar/39Ar zoic rocks of the Ouachita Mountain fold belt. The to orogenic belt is cored by the Benton uplift (Upper 0.7 ע ages for these cobbles range from92.4 Ma (Baksi 1997). The major surface ex- Cambrian to Lower Mississippian rocks) which is 0.9 ע 94.2 enclosed by Upper Mississippian and Lower Penn- sylvanian rocks, all of which have been strongly Manuscript received February 6, 2009; accepted June 19, 2009. folded (Arbenz 1989). The rocks of the Benton Up- 1 Department of Earth Sciences, University of Queensland, lift are predominately shales, clastic sandstones, Brisbane, Queensland 4072, Australia. cherts, and minor conglomerates (e.g., Ordovician [The Journal of Geology, 2009, volume 117, p. 615–626] ᭧ 2009 by The University of Chicago. All rights reserved. 0022-1376/2009/11706-0003$15.00. DOI: 10.1086/605779 615 This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:10:48 PM All use subject to JSTOR Terms and Conditions 616 G. N. EBY AND P. VASCONCELOS Figure 1. Regional setting for the Arkansas alkaline province and locations of the various intrusives. Modified from Morris (1987). Womble, Mazarn, and Collier shales, Crystal magnetic (Hildenbrand 1985) data indicate the ex- Mountain and Blakely Sandstone, and Mississip- istence of large subsurface intrusions (although pian/Devonian Arkansas Novaculite). Immediately of uncertain lithology). Carbonatite, carbonatite- east of the Ouachita Core, an erosional surface of pyroxenite breccias, pyroxenite, lamprophyre, ser- Middle Cretaceous age separates these rocks from pentinized olivine-bearing pyroxenite/peridotite, the overlying Upper Cretaceous to Holocene sedi- and partly altered felsic rocks were encountered in ments of the Mississippian embayment. drill holes in the central Arkansas area (Saline There is great petrographic diversity in the AAP County; Flohr and Howard 1994). (Morris 1987). Lithologies include diamond-bearing lamproite (Prairie Creek, madupitic olivine lam- Analytical Methods proite and phlogopite lamproite tuff; Mitchell and Bergman 1991, p. 46); lamproite (Dare Mine Knob); Fission-Track Ages. Apatite and titanite were sep- carbonatites and associated ijolites, jacupirangites, arated from crushed rock samples using standard nepheline syenites, and phonolites (Magnet Cove heavy liquid and magnetic separation techniques. and Potash Sulfur Springs); a variety of mafic and Apatite ages were determined using the population felsic dikes (V-intrusive and Benton dike swarm); method. Based on electron microprobe data, all the and the nepheline syenites and syenites of the apatites dated in this study are F-rich (F p Granite Mountain and Saline County intrusions. 1.39–3.28 wt%, and the Cl/[ClϩF] ratios, using Volumetrically, Granite Mountain and Saline weight percent abundances of the elements, are less County are the major intrusive complexes in the than 0.04). For each sample, the apatites were split AAP. Weathering of the central Arkansas syenites into two groups. One group was annealed for 6 h during the early Cenozoic produced the Arkansas at 520ЊC, irradiated, mounted, and polished. The bauxite deposits, the only commercial bauxite de- irradiation was done using pneumatic system 1 of posits in the United States. Besides the major in- the University of Massachusetts Lowell 1-Mw trusive complexes, thin carbonatite dikes have swimming-pool research reactor. The nominal flux been identified at Morrilton and Perryville (Mc- for this pneumatic system is1 # 1012 n cmϪ2 sϪ1, Cormick and Heathcote 1979), approximately 80 and the Cd ratio (relative to an Au monitor) is 24. km north of the Magnet Cove complex. Besides the The total neutron dose was monitored using NBS surface exposures, gravity (Hendricks 1988) and standard glass SRM962. The second set of unirra- This content downloaded from 23.235.32.0 on Wed, 21 Oct 2015 21:10:48 PM All use subject to JSTOR Terms and Conditions JournalofGeology ARKANSAS ALKALINE PROVINCE GEOCHRONOLOGY 617 diated grains was mounted and polished. Both sets leased during laser heating were removed by a of grains were simultaneously etched for 20–25 s Ϫ133ЊC cold trap and two C-50 SAES Zr-V-Fe get- at room temperature in 10% HNO3. Track densities ters. Argon isotopes in the cleaned gas fractions were subsequently determined using a #100 oil- were analyzed in an MAP 215-50 noble gas mass immersion objective. The zeta calibration method spectrometer. Full system blanks and air pipettes described by Hurford and Green (1982, 1983) and were determined before and after each sample. The Hurford (1990) was used to calculate the fission- data were corrected for atmospheric contamina- track ages. For this laboratory, the apatite z p tion, nucleogenic interferences, and mass discrim- -ination. All ages are reported using the K-decay val .10 ע 314 Titanite ages were determined using the external ues of (Steiger and Ja¨ger 1977). All errors are quoted detector method. Titanite grains were mounted in at the 2j confidence level (95%) and include the epoxy and polished to expose an internal surface. errors in the irradiation correction factors and the The grains were etched for approximately 1 h at error in J. The reported errors do not include the 120ЊC in a saturated NaOH solution. An external uncertainty in the age of the flux monitor or the mica detector was affixed to each grain mount. The uncertainty of the potassium decay constants. A samples were irradiated (using the same system as plateau age is defined as a sequence of three or more for apatite). Following irradiation, the mica detec- steps corresponding to at least 50% of the total 39Ar tors were etched for 12 min at room temperature released and whose age values are within 2j of the in concentrated HF. Fission-track densities for both mean value. An integrated age is the apparent age the grains and the detector were determined using obtained by the total gas released by a sample dur- a #100 oil-immersion objective. For this labora- ing the incremental-heating analyses.