HR: 15:30h
AN: GP73A-07    [Abstracts]
TI: Source of Magnetic Anisotropy in Andesitic Rocks: A case study from Java, Indonesia
AU: * Bijaksana, S
EM: satria@fi.itb.ac.id
AF: Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, Jalan Ganesa 10, Bandung, 40132, Indonesia
AU: Sastrawiharja, K
EM: medisas@staff.unnes.ac.id
AF: Department of Physics, Semarang State University, Jalan Raya Sekaran Gunungpati, Semarang, 50229, Indonesia
AU: Hodych, J P
EM: jhodych@mun.ca
AF: Department of Earth Sciences, Memorial University of Newfoundland, Alexander Murray Building 300 Prince Phillip Drive, St. John's, NL AIB 3X5,
AB: We have studied scores of intrusive and extrusive andesitic rocks from 21 sites in Java, Indonesia to gain insight into how magnetic anisotropy was developed in these rocks. The study includes measurement of anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic susceptibility (AAS), Curie temperature, magnetic hysteresis parameters, and XRF geochemical analyses. The percent anisotropy of AMS in these rocks varies from 1.4 to 8.4%, while the percent anisotropy of AAS varies from 15% to 69%. Magnetic lineation is predominant in ten sites, while magnetic foliation is predominant in the remaining eleven sites. No direct relationship was noted between magnetic lineation or foliation and structure. Measurement of Curie temperature shows that samples from some sites have a single magnetic phase while others show the presence of two to three magnetic phases; some of them have a Curie temperature as low as 208 C suggesting high Ti content. Assuming that the predominant magnetic mineral is magnetite, estimation of grain size made by plotting anhysteretic susceptibility as a function of low-field susceptibility suggests that only five sites are dominated by magnetic grains smaller than 1 micrometer. This was supported by measurement of hysteresis parameters. Cross-parameter analyses show that the number of magnetic phases in the rocks seems to affect the AMS but not the AAS parameters. Meanwhile variations in grain size affect both AMS and AAS. For samples with smaller magnetic grains, AMS degree of anisotropy increases as grain size increases while the AAS degree of anisotropy decreases as grain size increases. For samples with larger magnetic the degree of anisotropy the degree of anisotropy the degree of anisotropy grains, both AMS and AAS degree of anisotropy decrease as grain size increases. Geochemical analyses show that the degree of anisotropy tends to be larger for andesites with FeO content of less than 9%.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2009 Joint Assembly