INFERRING THE TIMING OF PORPHYROBLAST GROWTH IN THE ABSENCE OF CONTINUITY BETWEEN INCLUSION TRAILS AND MATRIX FOLIATIONS - CAN IT BE RELIABLY DONE

The timing of porphyroblast inclusion trails can be confidently interp reted relative to surrounding external foliations only where there is continuity between the two. Where this continuity is broken, timing is ambiguous. Where single or multiple growths of two or more different porphyroblastic minerals have occurred during a relatively complex def ormation history, the risk of misinterpreting the relative timing of p orphyroblast growth is high, and can lead to wrong inferences about pr essure-temperature-time-deformation (P-T-t-d) paths. Misinterpreting p orphyroblast timing can also have considerable consequences for determ ining rates of fabric evolution relative to changes in metamorphic con ditions. The effects of porphyroblast rotation vs nonrotation (relativ e to an externally fixed reference frame) on inferred P-T-t-d paths ar e poorly understood. However, the 'd' part of the path can differ cons iderably, depending on whether or not porphyroblasts are inferred to h ave rotated. The effect on the P-T-t part of the path depends on what effect inferences about porphyroblast rotation have on the inferred se quence of porphyroblast growth.