Thermomagnetic characteristics in late orogenic granites and gneisses of the southern Appalachian Piedmont

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Thermomagnetic curves have been obtained for samples from 25 granites and gneisses in Georgia, South Carolina, and North Carolina. This data set can be divided into two distinct curve types: Type I includes rocks which exhibit no distinct Curie points and a linear decrease in magnetization with increasing temperature; Type II curves exhibit a distinct Curie point over a range of temperatures expected for magnetite (500-580°C). Synthetic samples constructed from magnetite and hematite powder exhibit Type I behavior when the hematite to magnetite ratio is high (e.g. 15 : 1). Examination of polished sections shows relatively coarse-grained magnetite only in Type II rocks. We interpret our data to indicate that Type I thermomagnetic curves are dominated by relatively large hematite/magnetite ratios while Type II granites are characterized by relatively coarse-grained magnetite. Type I granite samples have low magnetic susceptibility values (less than 4 × 10-4 cgs) while most Type II granite samples have higher values. The Type I granites are invariably 18O-enriched whereas Type II granites typically exhibit low 18O/16O ratios. These relationships are consistent with previously reported correlations of susceptibility (generally indicative of magnetite content) and oxygen isotopic trends in the southern Appalachian Piedmont. © 1983.

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Earth and Planetary Science Letters

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