Granite is Never Isotropic: An Introduction to AMS Studies of Granitic Rocks

Bouchez, Jean‐Luc

doi:10.1007/978-94-017-1717-5_6

Cited by 306 publications

(340 citation statements)

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“…Although the general relation that K m increases with higher magnetite content is know (Bouchez, 1997;Rochette, 1987), we are the first, to our knowledge, to relate K m directly to the vol% of ferromagnetic minerals in ferromagnetic tonalites and granodiorites. However, the linear relationship be tween K m and vol% of magnetite grains presented herein does not take the composition of the magnetite into account, which could deviate from the pure Fe 3 O 4 composition when incorporating titanium (solid solution with ulvöspinel, Fe 2 TiO 4 ).…”

Section: Source Of Magnetic Anisotropy: Grains Vs Clustersmentioning

confidence: 92%

“…Archanjo et al, 2012;Bouchez, 1997;Cruden et al, 1999;Gleizes et al, 1998;Launeau and Cruden, 1998;López de Luchi et al, 2004;Petronis et al, 2012;Raposo et al, 2012;St Blanquat et al, 2001), lava flows (e.g. Cañón-Tapia and Coe, 2002;Cañón-Tapia, 2004;Loock et al, 2008), and dykes (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…The relation between AMS fabric and strain is not simple either : Arbaret et al (2013) showed that AMS and SPO fabrics in simple shear flow tend to stabilise parallel to the shear plane only at high strain and that the amount of strain required for fabric stabilisation depends on the aspect ratio of the particles and their initial orientation. Adding further complexity, the magnetic axes and the three-dimensional shape of the magnetic minerals are not always clearly associated (Bouchez, 1997).…”

Section: Introductionmentioning

confidence: 99%

“…It has only been proposed for rocks with K m smaller than 10 − 3 SI that K m correlates positively and linearly with the iron content of those rocks (Bouchez, 1997;Rochette, 1987).…”

Section: Introductionmentioning

confidence: 99%

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The relation between magnetite and silicate fabric in granitoids of the Adamello Batholith

et al. 2015

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Keywords:Rock fabrics in granites; X-ray micro-tomography; Anisotropy of magnetic susceptibility (AMS); 3D shape fabrics (SPO); Magnetic interactions AbstractThe link between the macroscopic silicate fabric and the magnetite-controlled AMS (anisotropy of magnetic susceptibility) fabric in ferromagnetic rocks was investigated through a comprehensive comparison between different fabric measurement techniques. Sample lithologies include tonalites and granodiorites from the Lago della Vacca Complex, Adamello Batholith, Italy. The datasets used to assess the link between subfabrics and the coherence between methods include: 1) macroscopic silicate fabric measured directly in the field; 2) macroscopic silicate fabric derived from image analysis (IA) of outcrop pictures and sample pictures; 3) shape-preferred orientations (SPO) of mafic silicates, 4) SPO of magnetite, and 5) calculated distribution of magnetite grains from computer-assisted highresolution X-ray tomography (X-ray CT) images; 6) fabrics derived from the AMS. Macroscopic mineral fabrics measured in the field agree with the IA results and with the SPO of mafic silicates obtained from the X-ray CT imaging. The X-ray CT results show that the SPO of the magnetite grains are consistent with the AMS data whereas the spatial distribution of the magnetite grains is less compatible with the AMS fabric. This implies that the AMS signal is mainly controlled by the shape of the magnetic carrier mineral rather than by the spatial arrangement of the magnetite grains. An exception is the presence of magnetite clusters. Furthermore, the SPO of mafic silicates and the SPO of the magnetite grains are consistent with the AMS data. Another finding of this study is that the magnetic susceptibility correlates linearly with the amount of magnetite in the samples. The coherent results obtained from a variety of methods reinforce the application of both AMS measurements and IA as robust tools to analyse fabrics in granitic intrusions.

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Section: Source Of Magnetic Anisotropy: Grains Vs Clustersmentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…It has only been proposed for rocks with K m smaller than 10 − 3 SI that K m correlates positively and linearly with the iron content of those rocks (Bouchez, 1997;Rochette, 1987).…”

Section: Introductionmentioning

confidence: 99%

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The relation between magnetite and silicate fabric in granitoids of the Adamello Batholith

et al. 2015

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“…Bouchez 1997). In particular, the lineation is often difficult to observe in the field because, unless they are significantly deformed, granitoids do not develop planar and linear fabrics that can be easily observed.…”

Section: Introductionmentioning

confidence: 99%

Gravity inversion, AMS and geochronological investigations of syntectonic granitic plutons in the southern part of the Variscan French Massif Central

Joly

Faure

Martelet

et al. 2009

Journal of Structural Geology

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Emplacement of the early Miocene Pinto Peak intrusion, Southwest Utah, USA

Petronis

O’Driscoll

2013

Geochem Geophys Geosyst

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[1] In this contribution, we report rock magnetic, petrographic, and anisotropy of magnetic susceptibility (AMS) data from the Pinto Peak intrusion, all of which bear on volcanic construction. Rock magnetic data indicate that the dominant magnetic mineral phase is low-Ti titanomagnetite of multidomain grain size, the composition of which varies spatially across the intrusion. The intrusion is a porphyritic andesite dominated by Ca-rich plagioclase (>60%) as well as biotite, amphibole, olivine, and opaque minerals. Reflected light petrography reveals mostly euhedral-subhedral (titano)magnetite crystals that often form clustered glomerocrysts and stringers of equant crystals, without exhibiting a consistent mineral alignment fabric. Moderate-to-shallow plunging prolate magnetic susceptibility ellipsoids dominate the northern part of the intrusion while steeply dipping/plunging magnetic susceptibility ellipsoids are generally restricted to the southern part of the intrusion. The vent facies rocks yield moderate-to-steep oblate susceptibility ellipsoids. We interpret the flow pattern in the north to reflect subhorizontal flow of magma, filling a tabular sheet-like body associated with propagation of the intrusion to the north. We argue that the southern part of the intrusion represents the ascent site of the magma rising to shallow crustal levels along a steep feeder system. The oblate magnetic fabrics in the vent area plausibly represent flattening against the conduit walls as evidenced by a weak planar flow foliation observed in the vent conduit rocks. On reaching shallow crustal levels, the magma deformed and uplifted the roof rocks leading to gravitational instability. As the slide mass released from the roof, an explosive eruption ensued resulting in the emplacement of the Rocks of Paradise tuff and associated effusive lava flows. Following eruption, magma pressure decreased and the magma drained northward forming the northern intrusive phase.

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Granite is Never Isotropic: An Introduction to AMS Studies of Granitic Rocks

Cited by 306 publications

References 0 publications

The relation between magnetite and silicate fabric in granitoids of the Adamello Batholith

The relation between magnetite and silicate fabric in granitoids of the Adamello Batholith

Gravity inversion, AMS and geochronological investigations of syntectonic granitic plutons in the southern part of the Variscan French Massif Central

Emplacement of the early Miocene Pinto Peak intrusion, Southwest Utah, USA

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