Diachronous cooling on both sides of a major strike slip fault in the Variscan Maures Massif (south-east France), as deduced from a detailed 40Ar/39Ar study
摘要
A detailed 40Ar/39Ar geochronological study was performed in the Maures Massif (SE France) to define the thermal history of this geochronologically poorly constrained segment of the Variscan belt. A total of 22 plateau ages were obtained on single grains and bulk samples of amphibole, muscovite and biotite extracted from 21 metamorphic and magmatic rocks sampled on both sides of the Grimaud Fault. Migmatites, micaschists, gneisses, pegmatites and granites displayed muscovite and biotite plateau ages ranging from 317.2±1.0 to 322.9±1.7Ma on the western side and from 300.2±0.6 to 306.0±2.4Ma to the east of the Grimaud fault. In the western block, amphiboles from amphibolites yielded plateau ages of 328.1±2.8 and 329.9±2.1Ma, whereas in the eastern block, amphiboles from amphibolite and magmatic bodies yielded variable ages ranging from 307.9±1.2 to 317.4±2.4Ma. These data clearly demonstrate distinct cooling histories between at least 330 and 300Ma, on both sides of the Grimaud Fault, which consequently appears as a major crustal fault during the late Variscan orogeny. As shown by the concordant (for each type of mineral) data obtained on biotite, muscovite, and amphibole from magmatic and metamorphic rocks, the two crustal blocks cooled down independently, each one homogeneously, in the range of temperatures 550–300°C. Two distinct periods of fast cooling appear, at 320Ma to the west and 305–300Ma to the east. These data are consistent with those previously obtained on other regions of the European Variscan belt.