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South-Central - 38th Annual Meeting (March 15–16, 2004)

Paper No. 3
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, (2)Department of Geological Sciences, Southern Methodist Univ, P.O. Box 750395, Dallas, TX 75275, griffin@utdallas.edu

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 (Fo83.9-Fo90.5) are typical of those from primitive mantle melts that have experienced minimal fractionation of mafic phases.  The phonolite olivines (Fo77.8-Fo82.5) exhibit evidence of Mg fractionation from a more chemically evolved melt. The clinopyroxenes from the nephelinites are dominated by Ca-rich diopsides (En41.8Fs10.8Wo47.4-En33.8Fs12.1Wo54.2). The phonolite clinopyroxenes are more compositionally diverse and include Ca-rich diopside, diopside and hedengbergite compositions (En36.0Fs12.8Wo51.3-En43.4Fs8.3Wo48.4-En13.5Fs39.8Wo46.7). 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.

 

Session No. 5--Booth# 15

General Posters III
Monday, 15 March 2004: 8:00 AM-5:00 PM
Geological Society of America Abstracts with Programs. Vol. 36, No. 1, p.5
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