FOLDING AND GRANITE EMPLACEMENT INFERRED FROM STRUCTURAL, STRAIN, TEMAND GRAVIMETRIC ANALYSES - THE CASE-STUDY OF THE TULLE ANTIFORM, SW FRENCH MASSIF-CENTRAL

In the South Limousin (French Massif Central), the Tulle anticline is a kilometre-scale, upright, open fold which re-folds an already foliat ed Variscan stack of nappes. Three main granodioritic plutons associat ed with numerous folded dykes and boudinaged sills crop out in the cor e of the Tulle anticline. Microstructural study, including quartz (c)- axis petrofabrics and transmission electron microscope (TEM) analysis allows us to recognize the timing of deformation during pluton emplace ment and cooling. The earliest fabric is a pre-full crystallization de formation. The second stage corresponds to a high-temperature (ca 650 degrees C) solid-state deformation. The last deformation is responsibl e for the (c)-axis preferred orientation at low temperature. Several m ethods of strain analysis applied in various mineral phases show that pluton emplacement is coeval to a constrictional strain. This strain i s characterized by a NW-SE maximum stretching axis (X) parallel to the Tulle anticline axis and an intermediate. NE-SW-trending, shortening axis(Y) also responsible for large open folding of the host rocks. Gra vimetric modelling allows us to determine a laccolithic shape for the plutons. Hot Stokes diapirism is ruled out and the hypothesis of grani tic magma channelling through dykes is preferred. A laccolith emplacem ent model based on a syn-folding decollement level is proposed. (C) 19 98 Elsevier Science Ltd. All rights reserved.