Shape, size, spatial distribution and composition of garnet crystals in highly deformed gneiss of the Otter Lake area, Quebec, and a model for garnet crystallization

摘要

Garnet-biotite-(sillimanite) gneiss (similar to 700 degrees C, 7 kbar) of the Otter Lake area in the Western Grenville Province (Canadian Shield) occurs as granitic gneiss (group 4) that forms a large part of the Otter Complex, and as widely distributed, more heterogenous metasedimentary gneiss (group 2). In one sample of group 4 gneiss (Qtz(25) Pl(34) Kfs(28) Bt(10) Grt(2.5) Sil(1)) the true diameter (determined by serial grinding) of subhedral garnet crystals ranges from 0.2 to 3.0 mm, with a mode at 1.0 mm. Nearest-neighbour measurements in this sample, and in surfaces of nine additional samples (all < 5% garnet) confirm that garnet crystals are distributed mainly at random; slight clustering was detected in two samples. In one sample of group 4 gneiss, microprobe analyses on sections through crystal centres (obtained by serial slicing), reveal that small crystals and margins to large crystals contain more Fe and Mn and less Mg than the broad central regions of large crystals. Based on these and previous results, together with theoretical considerations, a crystallization model is proposed, in which, (i) garnet was produced by the continuous reaction, Ms + Bt + Qtz -> Grt + Kfs + H2O, (ii) nucleation occurred by the random selection of randomly distributed Ms-Bt-Qtz triple junctions, (iii) the rate of linear growth remained constant, and (iv) as temperature increased, the rate of nucleation first increased slowly, then remained nearly constant, and finally declined. Within-population compositional homogenization was followed, on cooling, by local Fe-Mg-Mn exchange with biotite.