Geosciences (Apr 2023)
Rock Magnetism and Magnetic Fabric Study of the Icelandite and Rhyodacite Long Volcanic Sequence at Mauna Kuwale, Wai’anae Volcano, Oahu, Hawaii, USA
Abstract
In order to understand further the emplacement (i.e., volcanic growth) of 22 Icelandite and 3 Rhyodacite cooling units in one of the long volcanic sequences known as Mauna Kuwale of the Wai’anae volcano (ca. 3.3 Ma), Oahu Hawaii we have conducted appropriate rock magnetic experiments described below as well as anisotropy of magnetic susceptibility (AMS) studies of such 25 units. We have undertaken rock magnetic investigations such as continuous and partial thermo-magnetic cycles of low field magnetic susceptibility versus temperature dependence, (k-T) curves experiments. We classified the k-T heating-cooling dependence of susceptibility in three groups A, B and C. Type A: yielded two components of titano-magnetite with a predominat Ti rich phase and occasionally a relevant magnetite component phase. Type B: samples are characterized by Ti poor magnetites. Magnetite dominates as the main magnetic carrier. Type C: k-T curves show one single phase of titanomagnetite, and Ti poor magnetite. The coercivity or remanence, determined by back field magnetization is always −3 SI). Magnetic anisotropy/magnetic fabric is used as a tool in rock fabric analyses to investigate the preferred orientation of magnetic minerals in rocks. Magnetic anisotropy is low on all (measured) flows from the Icelandite cooling units from 1 to 17 (mean P’ = 1.010), but becomes noticeably distinct and high in rhyodacite cooling units 23, 24 and 25 (mean P’ = 1.074). Four units show a magnetic fabric with k3 axes vertical to sub-vertical which may be denoted as normal for the horizontal to sub horizontal units. Two Icelandite cooling units display oblate shapes and two other cooling units triaxial shapes. K1 axes are horizontal but point in different directions, i.e., NE and NW. Remaining cooling units show different magnetic fabric. Units 17, 23, 24 and 25, despite important variations in anisotropy (low for units 25 and high for units 23 and 24) and shape of ellipsoid (oblate in cooling unit 23, prolate in 24 and triaxial in 25) the k3 axes show the same orientation, SW to SSW dipping around 45° and a very steady magnetic lineation azimuth NW nearly horizontal to sub horizontal. The magnetic mineralogy and magnetic fabric indicate that both the Icelandite and Rhyodacite cooling units the magmatic evolution during the shield stage of the entire Wai’anae volcano and that such growth was not affected by tectonic deformation.
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