During burial of clay-rich sediments, magnetic minerals, just as other minerals, are continuously produced, altered, and dissolved. In shales, from early burial to anchi-metamorphism (Tburial< 250°C), magnetite is the main magnetic mineral. In slates, it is known that the dual breakdown of magnetite and pyrite trigger the formation of pyrrhotite for burial temperature close to 300°C (greenschist metamorphic grade). We propose to bracket the range of temperature of magnetite and pyrite breakdown using in combination magnetic studies and burial temperature obtained by Raman spectroscopy. Samples are late Cretaceous slates (burial temperature 250°-600°C) from Pyrenees. The identification of magnetite and pyrrhotite is firmly constrained by the identification of Verwey ( 120 K) and Besnus ( 32K) magnetic transition. Burial temperatures are estimated within ±30°C. Results show: 1) Magnetite is dominant for Tburial<330°C, with evidence of fine pyrrhotite in the superparamagnetic to single domain states. Magnetization is pretty low (<10-4 Am2/kg). 2) An assemblage of magnetite and pyrrhotite is observed for the range 330°C< Tburial <360°C. 3) Pyrrhotite is the only ferrimagnetic mineral detected for the range 360°C< Tburial <600°C. Magnetization displays a wide range (10-4-10-1 Am2/kg) in the pyrrhotite windows. There is therefore a high contrasting magnitude of magnetization of shales vs. slates. In shales, the remanence carried by magnetite is essentially a chemical remanent magnetization imprinted during peak burial. In contrast, the remanence carried by pyrrhotite is essentially a thermo remanent magnetization blocked in during the uplift of metamorphic units.