ELECTRON MICROPROBE ANALYSIS OF OLIVINE AND CLINOPYROXENE FROM NEPHELINITES AND PHONOLITES OF THE BALCONES IGNEOUS PROVINCE OF TEXAS

Olivine and clinopyroxene grains (n=78) from samples (n=12) collected in the Balcones igneous province of Central Texas were analyzed using wavelength dispersive electron microprobe techniques. Lithologies of the analyzed samples include melilite olivine nephelinite, olivine nephelinite and phonolite. Nephelinite olivines (Fo_83.9-Fo_90.5) are typical of those from primitive mantle melts that have experienced minimal fractionation of mafic phases. The phonolite olivines (Fo_77.8-Fo_82.5) exhibit evidence of Mg fractionation from a more chemically evolved melt. The clinopyroxenes from the nephelinites are dominated by Ca-rich diopsides (En_41.8Fs_10.8Wo_47.4-En_33.8Fs_12.1Wo_54.2). The phonolite clinopyroxenes are more compositionally diverse and include Ca-rich diopside, diopside and hedengbergite compositions (En_36.0Fs_12.8Wo_51.3-En_43.4Fs_8.3Wo_48.4-En_13.5Fs_39.8Wo_46.7). The Mg depletion in the phonolite olivines along with the phonolite clinopyroxene compositional trends support the idea that the Balcones province phonolites are later stage fractionates of a nephelinite parental magma.

Presentation Time: 8:00 AM-5:00 PM
ELECTRON MICROPROBE ANALYSIS OF OLIVINE AND CLINOPYROXENE FROM NEPHELINITES AND PHONOLITES OF THE BALCONES IGNEOUS PROVINCE OF TEXAS
GRIFFIN, William R.¹, BERGMAN, Steven C.¹, LEYBOURNE, M.I.¹, and BEAVERS, Roy², (1) Department of Geosciences, Univ of Texas at Dallas, P.O. Box 830688, FO21, Richardson, TX 75002, griffin@utdallas.edu, (2) Department of Geological Sciences, Southern Methodist Univ, P.O. Box 750395, Dallas, TX 75275 Olivine and clinopyroxene grains (n=78) from samples (n=12) collected in the Balcones igneous province of Central Texas were analyzed using wavelength dispersive electron microprobe techniques. Lithologies of the analyzed samples include melilite olivine nephelinite, olivine nephelinite and phonolite. Nephelinite olivines (Fo_83.9-Fo_90.5) are typical of those from primitive mantle melts that have experienced minimal fractionation of mafic phases. The phonolite olivines (Fo_77.8-Fo_82.5) exhibit evidence of Mg fractionation from a more chemically evolved melt. The clinopyroxenes from the nephelinites are dominated by Ca-rich diopsides (En_41.8Fs_10.8Wo_47.4-En_33.8Fs_12.1Wo_54.2). The phonolite clinopyroxenes are more compositionally diverse and include Ca-rich diopside, diopside and hedengbergite compositions (En_36.0Fs_12.8Wo_51.3-En_43.4Fs_8.3Wo_48.4-En_13.5Fs_39.8Wo_46.7). The Mg depletion in the phonolite olivines along with the phonolite clinopyroxene compositional trends support the idea that the Balcones province phonolites are later stage fractionates of a nephelinite parental magma.
South-Central - 38th Annual Meeting (March 15–16, 2004) General Information for this Meeting

Session No. 5--Booth# 15 General Posters III Texas A&M University: Geology Building, Room 13 8:00 AM-5:00 PM, Monday, March 15, 2004 Geological Society of America Abstracts with Programs, Vol. 36, No. 1, p. 5
© Copyright 2004 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.

South-Central - 38th Annual Meeting (March 15–16, 2004)
Paper No. 5-3