SUBGLACIAL COMMINUTION IN TILL - EVIDENCE FROM MICROFABRIC STUDIES AND GRAIN-SIZE DISTRIBUTIONS

Quartz, feldspar and chlorite, the principal minerals in a basal till from the Halasi River catchment in the Altay Mountains, northwestern C hina, are present in approximately equal concentrations in the coarse fraction of the till, 1.0-0.125 mm. Quartz concentrations are signific antly higher than those of the other two minerals in the 0.125-0.016 m m size range. Feldspar and chlorite concentrations are higher than tho se of quartz in the finest fraction. Quartz has a strong preferred mod e at 0.063-0.032 mm. Feldspar and chlorite have two weak modes in the sill-size range, one between 0.063 and 0.032 mm and the other between 0.016 and 0.004 mm. Thin sections of oriented impregnated samples were used to study crushing and abrasion. Over 2700 daughter particles wer e identified as products of comminution of 925 parent grains. Quartz a nd feldspar are most likely to be broken into two particles of roughly equal size, as are fine chlorite grains. However, owing to their weak ness and cleavage, larger grains of chlorite tend to be split into mor e than two daughter particles. Sizes of the daughter grains were measu red and sizes of the original parent grains were estimated. Mean paren t grain-sizes for quartz, feldspar and chlorite are 0.129, 0.078 and 0 .059 mm, respectively, whereas mean daughter grain-sizes are 0.068, 0. 041 and 0.024 mm, respectively. The greater percentage reduction in th e size of chlorite reflects its tendency to break into more than two d aughter particles. Most grains tend to be crushed. Only a few large pa rticles seem to have suffered from abrasion.