Magnetite microexsolutions in silicate and magmatic flow fabric of the Goyozan granitoid (NE Japan): Significance of partial remanence anisotropy

Abstract

[1] Anisotropy of magnetic susceptibility (AMS) has been widely used to infer magmatic flow patterns of granitoids where an appropriate AMS axis is parallel to an alignment of mafic minerals or magnetite. The magmatic flow fabric in Cretaceous granitic plutons from northeastern Japan was verified using an analysis of anisotropy of partial anhysteretic remanent magnetization (ApARM) which further isolates the magnetite subfabrics according to magnetite grain size. The preferred orientation of polysynthetic twins in plagioclase laths and clinopyroxene is discordant with the bulk AMS fabric along outer marginal zones of the granitoid, as shown by image analysis of microphotographs from thin sections cut in orthogonal planes. This suggests that the uncorroborated use of bulk AMS to detect flow fabric in granitoids has risks. Scanning electron microscopy (SEM) reveals that submicroscopic, needle-shaped magnetite inclusions exsolved in anhedral plagioclase and clinopyroxene may explain such anomalous exceptions to the validity of AMS fabrics. Our ApARM measurements show that the ApARM alignment of relatively high-coercive, submicroscopic magnetite inclusions is concordant to the linear orientation of anhedral plagioclase and clinopyroxene. The combination of SEM, AMS, and ApARM was required to confirm the magmatic and submagmatic flow pattern of granitoids in this study and is generally preferable to the use of AMS alone.Citation: Usui, Y., N. Nakamura, and T. Yoshida (2006), Magnetite microexsolutions in silicate and magmatic flow fabric of the Goyozan granitoid (NE Japan): Significance of partial remanence anisotropy,

Paleointensity estimates from oceanic gabbros: Effects of hydrothermal alteration and cooling rate

“…The present study has revealed that exsolved magnetite in oceanic gabbros provide the expected paleointensity, and that FORC analysis is effective to find samples dominated by exsolved magnetite. Exsolved magnetite seems to be common in oceanic gabbros (Davies, 1981), and it has been also reported from continental plutonic and metamorphic rocks (e.g., Selkin et al, 2000;Usui et al, 2006;Wenk et al, 2011). In particular, oceanic gabbros of various geologic ages are preserved on land as ophiolites, and some of them have been successfully investigated for paleointensity (Granot et al, 2007).…”

“…Targeted measurements of magnetic inclusions in silicate crystals may exclude the contamination (Cottrell and Tarduno, 1999;Tarduno et al, 2006). Several studies have shown that plagioclase or clinopyroxene in some plutonic rocks contain tiny, exsolved magnetite inclusions which are near stoichiometric and magnetically stable (Davies, 1981;Feinberg et al, 2005;Usui et al, 2006). However, the strong texture of such a magnetite within a host mineral produces a large magnetic anisotropy (Feinberg et al, 2006).…”

“…However, the strong texture of such a magnetite within a host mineral produces a large magnetic anisotropy (Feinberg et al, 2006). This makes paleointensity experiments using single silicate crystals difficult when they contain exsolved magnetite (Usui and Nakamura, 2009), whilst a nearrandom texture of host minerals would reduce the magnetic anisotropy at a whole rock scale (Usui et al, 2006). If one uses whole rock samples, it is necessary to discriminate samples whose magnetic signals are dominated by secondary magnetic minerals.…”

Nonlinear thermoremanence corrections for Thellier paleointensity experiments on single plagioclase crystals with exsolved magnetites: a case study for the Cretaceous Normal Superchron

“…Sample GG27 is a medium-grained biotite-hornblende granodiorite, located in the southern marginal zone of the complex. Microscopic observation of thin sections revealed the presence of large euhedral magnetites associated with clinopyroxenes and hornblendes and of magnetite microexolutions in plagioclase (Usui et al, 2006). Magnetic data also showed that GG27 has a coercivity spectrum from distinct grain-size distribution of magnetites, with abundant very fine-grained and high coercivity fraction together with a large-grained and low coercive fraction.…”

“…Plagioclase crystals were extracted from crushed granidoid samples taken from single site of the Goyozan Plutonic Complex in the South Kitakami massif, Northeast Japan (GG27 sample of Usui et al, 2006). Using samples from only one site, we restrict our attention to a proof of technique rather than to an estimation of true paleointensity here.…”