FIA measurements

The measurement of a FIA is achieved by cutting a minimum of eight vertically oriented thin sections around the compass from each rock sample to locate the switch in inclusion trail asymmetry (clockwise or anticlockwise) within the porphyroblasts (Fig. 6a and b). Where the FIA trends vary from the core to the rim of the porphyroblasts, a relative timing and thus a FIA succession can be established (e.g. Bell et al. 1995, 1998).

Figure 6. The FIAs method

The FIAs method

(a) Sketch illustrating the method developed by Bell (1995, 1998) by which the trends of FIA are measured. This technique uses changes in asymmetries of inclusion trails in a porphyroblast, when viewed in a consistent direction for successive striking vertical thin sections. The range of inclusion trail geometries expected in thin sections of varying orientation along a single FIA, which is between 0 and 40 in this case, is shown. The inclusion surfaces marked on thin sections represent the geometry of the inclusion trails within the porphyroblast. Thin section orientation is marked as barbed arrow. The position of eye ball indicates that the geometry is viewed from that direction. (b) The 3-D sketch illustrates a change in FIA sets.