Scattering of magnetic fabrics in the Cambrian alkaline granite of Meruoca (Ceará state, northeastern Brazil)

Abstract

Anisotropy of magnetic susceptibility (AMS) applied to an alkaline granite from Meruoca (NE Brazil) recorded weak anisotropies, typically below 4%, and a considerable dispersion of the AMS axes. Red-clouded feldspars and clots of metasomatic minerals enclosed in magmatic crystals indicate that hydrothermal fluids altered the granite. U-Pb isotopic data show high-common Pb on zircons but allowed the calculation of a mean SHRIMP age of 523 ± 9 Ma attributed to the magmatic crystallization. Growth of fine oxides by late fluid-rock interactions was responsible for the scattering of AMS. Rock magnetic data indicate they consist mainly of an oxidized magnetite and (titano)hematite. Shape preferred orientation of mafic aggregates measured in granite quarries shows that the pluton preserves a gently dipping magmatic foliation. AMS in some quarries with a well-defined magmatic fabric, however, remains highly dispersed. When AMS mimics the mafic shape fabric, only magnetic foliations share a common orientation. Locally, AMS grounded in coarse Ti-poor magnetite associated with titanite develops a consistent subhorizontal oblate fabric that agrees with tectonic models suggesting that the cupola of the pluton has been exposed by erosion.

Hydrothermally-induced changes in mineralogy and magnetic properties of oxidized A-type granites

“…10c). Archanjo et al (2009) also identified titanohematite with Ti-poor and Ti-rich exsolutions as small grains in chloritized biotite.…”

“…The IRM acquisition curves for the different facies of the Meruoca granite are given in Archanjo et al (2009). The greenish granite can be saturated at fields lower than 130 mT, hence only a soft (low coercivity) component is present.…”

“…Actually, the change in color only appears when there is enough hematite and/or iron hydroxides. The first (Archanjo et al, 2009) 0.1 appearance of hematite may sometimes occur before any substantial decrease of the magnetic susceptibility.…”

“…Comparison of the AMS fabric with the mineral fabric acquired by an independent method may be also worth considering. This comparison has been attempted by Archanjo et al (2009) for the Meruoca pluton, where mafic minerals define a subhorizontal magmatic foliation, while AMS of the reddish units show considerable scattering. Conversely, for the non-altered green to gray granites they found a good correspondence between magnetic and magmatic foliations.…”

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

“…Consequently, in this type of magmatic rocks, AMS (magnetite subfabric) and IA (subfabric of silicates) are complementary, and their combination has proven to be highly useful to reconstruct the emplacement dynamics of intrusive magmas (e.g. Archanjo et al, 2002;Archanjo et al, 2009Archanjo et al, , 2012Diot et al, 2003) and to infer flow characteristics of ignimbrites and basaltic lava flows (e.g. Boiron, 2011;Gountié Dedzo et al, 2011).…”