Garnet crystal growth in sheared metapelites (southern Calabria – Italy): relationships between isolated porphyroblasts and coalescing euhedral crystals

Three different generations of garnet (Type 1a, Type 1b and Type 2) occur in sheared metapelite of the Cardeto Metamorphic Complex (CMC), cropping out in the southern sector of the Aspromonte Massif (southern Calabria, Italy). Main differences are their size, chemical trends, inclusion trails geometry, and crosscutting relationships with different foliations. All of these aspects were used to discriminate between garnet typologies. The CMC was metamorphosed under greenschist to low-amphibolite facies conditions. The peculiar feature of the whole complex is the presence of a pervasive mylonitic foliation. Alternation of quartz and mica domains, the latter essentially consisting of chlorite, muscovite and subordinately of biotite, gives the main schistosity. Besides these minerals garnet, plagioclase, epidote and amphibole complete the typical rock assemblage. Microstructural evidences associated with mineral-chemical data suggest multistage garnet growth. The first garnet generation (Type 1a) is represented by pre-mylonitic porphyroblasts (up to 8 mm in diameter), usually characterized by plane-parallel inclusion trails. The second generation consists of syn-mylonitic garnets (Type 1b), characterized by spiral-shaped inclusion trails, which give to them the classical snowball appearance. Internal foliations are commonly defined by aligned grains of quartz and ilmenite. A multistage growth was recognized for these garnets by means of both compositional traverses and X-ray maps revealing concentric zonation patterns. The second garnet generation (Type 2) consists of smaller euhedral garnets, usually 100 μm in diameter (occasionally up to 400 μm). They typically occur as several adjacent crystals coalescing around Type 1a and 1b garnets. In samples the outer edge of garnet porphyroblasts (Type 1a and 1b) is the indentation of the adjoining borders of many small idioblastic coalescing grains (Type 2). Layers of small euhedral garnets, concentrically chemically zoned, parallel to the mylonitic foliation were also observed. They could have been developed through different processes: (1) mechanical alignment of already existing Type 2 garnet grains due to the shear deformation or (2) they represent a further stage of crystallization prevalently localized into small domains parallel to the main foliation, favoured by a high mobility elements flux along these surfaces.The process of coalesced euhedral garnets over pre-existing garnet porphyroblasts, here described, could be a widespread crystallization mechanism inside shear zones. The elevated fluid concentration typically present within shear zones could promote the sustenance of chemical elements which support either overgrowth or multistage crystal growth processes