Transformation of cataclastically deformed rocks to pseudotachylyte by pervasion of frictional melt: inferences from clast-size analysis

摘要

Optical contrast between clasts and surrounding dark-coloured matrix in thin sections has been used in a computer-aided method which has been applied for clast-size analysis of pseudotachylyte exposed in a 25-km-long brittle deformation zone in the Precambrian terrain of western India. While the number of clasts (N_2) equal to or coarser than size u shows a power-law distribution (N_2 #alpha# u~(-D)), the area (A_1) occupied by the clasts of different size fractions shows a log-normal distribution, The clast-size distribution pattern indicates transformation of cataclastically deformed rocks to pseudotachylytes by pervasion of a rock melt generated at the frictional slip planes. It is inferred that the melt pervaded the permeable cataclastically damaged zones and so occupied the intergranular space. This melt, mixed with the comminuted material, flowed further as a slurry to form the pseudotachylyte veins. The clast-laden final product retained the grain-size distribution pattern of the parent comminuted material with minor modifications. Fusion of clast rinds due to heat transfer from the melt slightly modified the initial grain-size distribution pattern. Because of the short duration of the heat-transfer process, combined with the low conductivity and high specific heat of rocks, the temperature at the centre of the coarse grains remained at the initial temperature T_i and reduction w of radius r by fusion was a negligible fraction of r. As a consequence, the population of coarse clasts retained the pre-fusion size distribution pattern. In contrast, w/r of the smaller grains was considerable. The initial grain-size distribution pattern did not survive in the finer fraction of the clasts. This difference in thermal response of the smaller versus larger grains to the heat-transfer process is revealed in clast-size distribution graphs for the pseudotachylytes. Moreover, the observed proportion of matrix can not be explained by cataclasis alone. The extent of comminution in the parent material and size of the openings, through which the clast-laden melt flowed to form veins, controlled the clast/matrix ratio in the pseudotachylytes.