Inferring the timing of porphyroblast growth in the absence of continuity between inclusion trails and matrix foliations: can it be reliably done?

Abstract

The timing of porphyroblast inclusion trails can be confidently interpreted 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 deformation history, the risk of misinterpreting the relative timing of porphyroblast growth is high, and can lead to wrong inferences about pressure-temperature-time-deformation ( P-T-t-d) paths. Misinterpreting porphyroblast timing can also have considerable consequences for determining rates of fabric evolution relative to changes in metamorphic conditions. The effects of porphyroblast rotation vs non-rotation (relative to an externally fixed reference frame) on inferred P-T-t-d paths are poorly understood. However, the ‘ d’ part of the path can differ considerably, depending on whether or not porphyroblasts are inferred to have rotated. The effect on the P-T-t part of the path depends on what effect inferences about porphyroblast rotation have on the inferred sequence of porphyroblast growth.