Microtextures of opaque inclusions: their use as indicators for hiatuses during garnet porphyroblast growth

Within an analysed garnet porphyroblast, opaque inclusions imaged with the backscatter facility of a scanning electron microscope show different micro textures depending on their position within the porphyroblast. Three differ ent zones can been distinguished: Zone 1 contains a Ti-rich magnetite that has decomposed to a symplectite of fine and narrowly spaced exsolution lame llae of ilmenite and magnetite. Zone 2 shows a Ti-rich magnetite symplectit e with thicker and more widely spaced exsolution lamellae of ilmenite and m agnetite. Within zone 3, Ti-rich magnetite symplectite has totally been rep laced by recrystallized magnetite crystals bordered by a thin ilmenite rim. Similar microtextures within ulvospinel-rich magnetite have elsewhere show n to be the result of an increase in oxidation and rate of diffusion. Durin g metamorphism of metapelites, such an increase can be reasonably envisaged because of dehydration reactions progressing during rising temperatures, a nd this has occurred during the overgrowth of the three different microtext ures by the garnet porphyroblast. Because the microtextures are homogeneous within the three different zones, it is deduced that the oxidation reactio n rate of the opaque inclusions was substantially lower than the garnet gro wth rate. As a consequence, hiatuses in the garnet growth history must have occurred between the evolution from one microtexture to the next. A compar ison between the inclusion trail geometry and the microtextural zone bounda ries shows a perfect coincidence between these and the sites where inclusio n trails become strongly deflected and truncated. This correlation confirms that, in the studied case, sharp microstructural boundaries (as truncation zones or deflection zones) coincide with growth hiatuses. The study theref ore highlights the potential use of opaque inclusions to confirm or reject the occurrence of growth hiatuses within garnet porphyroblasts, especially in cases where discontinuities in the inclusion trail patterns are otherwis e arbitrarily associated with growth hiatuses.