19 Undergraduate Research (Posters)

Friday, 18 March 2005
8:00 AM - 12:00 PM

PETROLOGY OF THE NINE TIMES AMPHIBOLITE-ULTRAMAFIC BODY FROM THE SUNSET 7.5-MINUTE QUADRANGLE, WESTERN INNER PIEDMONT OF SOUTH CAROLINA

Paper number: 19-14
Program: THEME SESSIONS
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The lenticular Nine Times amphibolite-ultramafic body (3.2 km long by 0.1-0.3 km wide) lies in the western Sunset quadrangle. An ultramafic portion forms the core of the body with an overhanging amphibolite at either end. The body is surrounded by Table Rock biotite gneiss and lies within the polydeformed granitic and mafic rocks of the Walhalla nappe. It is believed to correlate with an amphibolite-ultramafic layer in the Table Rock quadrangle ~12 km to the northeast.

Petrographic analysis of 10 ultramafic samples reveals the major mineralogy is about 60% clinoamphibole and 40% chlorite, with minor oxide minerals. Garnet is a minor accessory mineral in two samples but was not observed in thin section. All samples show a well-developed foliation defined by aligned chlorites and amphibole crystals in alternating bands, with a crosscutting schistosity. Two of the ultramafic samples show large relict olivine crystals with embayed margins, surrounded by chlorite and amphibole. The olivine is biaxial (+) with a high 2V, indicating a Mg-rich composition. Relict olivine grains are fractured and show extensive oxide exsolution. Chlorite is also Mg-rich, with pale green pleochroism and abnormal brown interference colors. Metamorphic amphiboles occur in magnesio-hornblende and edentic-hornblende compositional varieties. Magnesio-hornblende grains are pleochroic (colorless to pale green) with maximum lower 2nd order interference colors, display high 2V (~60-70°), and are biaxial (-). Edentic-hornblende grains form longer prisms, have pale blue-green pleochroism, middle 2nd order interference colors, a high 2V (~60-70°), and are biaxial (-). Edentic-hornblende grains have reacted margins and abundant oxide exsolution. Further petrography and microprobe analysis will examine mineral chemistry and reaction relations in conjunction with bulk rock chemical analysis.