Growth and rotation history of garnet porphyroblasts with inclusion spirals in a Karakoram Schist

Idioblastic garnet porphyroblasts in the metapelitic layers of a Cenozoic garnet-staurolite-biotite-muscovite-plagioclase schist from the Hunza valley, Karakoram Mountains, Pakistan, contain both quartz and graphite inclusion spirals. Growth-zones within the crystals enable reconstruction of the growth and rotation history of each porphyroblast. All the observed microstructural features are adequately accounted for by the model of Schoneveld (1977), which involves growth of the porphyroblast during rotation relative to the matrix. Apparent rotation angles up to 540° have been found. In contrast, xenoblastic to sub-idioblastic garnet porphyroblasts in adjacent metapsammitic layers have only slightly curved inclusion trails. The quartz spirals appear to have developed by the garnet overgrowing quartz concentrated in "pressure-shadow" domains around the growing porphyroblasts. However, none of the quartz spirals reach the margins of the final crystal outline, and the present "pressure-shadow" domains contain little or no quartz. Furthermore, microprobe analyses of growth-zones within the garnet crystals show a relatively abrupt change in the concentrations of MnO, MgO and FeO across a boundary coinciding approximately with the disappearance of quartz from the inclusion spirals. These changes may be related to a change in metamorphic reactions during garnet growth.