GRANITE MAGNETIC TYPES : A REVIEW AND IMPLICATIONS FOR AMS STUDIES

The anisotropy of magnetic susceptibility (AMS) is a strain marker in granites and has greatly contributed to the debate on pluton emplacement studies. In most cases, the source of the magnetic susceptibility (K) and of the AMS are determined with different techniques. The aims of this review are to propose a new granite magnetic typology and to investigate its implications for AMS studies.

The early classification in Ilmenite- and Magnetite-series led to the distinction between paramagnetic types with K < 250.10^-6 SI and ferromagnetic types with K > 250.10^-6 SI. This dichotomy has the disadvantage of blurring the source of K, particularly when K originates in part from ferrimagnetic trace amounts. We define a new ferromagnetic index Ncferro as the ratio c ferro / c bulk. A granite is ferromagnetic when Ncferro > 0.5 and paramagnetic when Ncferro < 0.5. The new typology reviews > 70 case studies of granites of different ages, compositions and settings. Three tests, using rock chemistry, thermomagnetic and hysteresis properties are sufficient to identify the dominant magnetic susceptibility carrier. Seven main types of granites are defined by their magnetic and mineralogical features.

Ferromagnetic granites are the most common in Archean and Proterozoic terranes. Paramagnetic granites are more important in Caledonian orogens and dominate Variscan orogens. Cretaceous and younger subduction-related batholiths display two granite belts with oxidized (Mt) and reduced (Ilm) types. It has been suggested that organic material subducted in the trench contributed to the oxygen fugacity control. I-type, H-type, A-type and some S-type granites are mostly ferromagnetic. Paramagnetic granites correspond closely to peraluminous and reduced calc-alkaline compositions and, in rare cases, to reduced A-type granites.

The significance of AMS data is ambiguous when magnetite inclusions occur within mafic silicates or when AMS carriers crystallize interstitially in the matrix.