Magnetic susceptibility and geochemistry of Variscan West Carpathian granites: implications for tectonic setting

Abstract

Granites usually display a bimodal distribution of their magnetic susceptibility. One mode corresponds to susceptibilities of the order of 10 −5–10 −4 and the other one to those of the order of 10 −3–10 −2 [SI]. The former mode, which corresponds to ilmenite-bearing granites, is often equated with the S (sedimental) type, while the latter, which corresponds to magnetite-bearing granite, is equated with the I (igneous) type. In the West Carpathians, isotope geochemistry discriminates two groups of Variscan granites: (1) the older group (350–330 Ma) is peraluminous, from two-mica granites to granodiorites, carrying monazite and ilmenite; it resembles to common S-type and/or Ilmenite Series granite, (2) the younger group (310–300 Ma) is metaluminous to subaluminous, corresponding to biotite tonalites to granodiorites, carrying allanite and magnetite; it can be compared to the I-type and/or Magnetite Series granites. Magnetic susceptibilities of the West Carpathian granites are in general rather low, in the order of 10 −4, hence corresponding to the typical values of the S type granites. A minority of specimens have susceptibilities in the order of 10 −5 and a few specimens have susceptibilities higher, in the order of 10 −3. This is in apparent contradiction with the granite origin as revealed geochemically. Unusual redox conditions controlled by the tectonic setting are possible explanations of this discrepancy.