Roundness of clasts in pseudotachylytes and cataclastic rocks as an indicator of frictional melting

We examine the roundness (Rd) of clasts derived from fault rocks and discus s its possible application as an indicator for fracturing and chipping or f rictional melting. The roundness of quartz and feldspathic clasts derived f rom four frictional melting-originated pseudotachylytes and one crushing wi thout melting-originated pseudotachylyte as well as three cataclastic rocks was measured. Between 230 and 270 clasts > 10 mu m in size were measured f or each sample. The analyses show that Rd is lower than 0.4 in all the clas ts included in the cataclastic rocks and crushing-originated pseudotachylyt e, whereas it varies from 0.1 to 1.0 and 35-90% of clasts have a roundness > 0.4 in the melting-originated pseudotachylyte. It is suggested that the c lasts having a roundness > 0.4 were rounded by frictional melting rather th an by fracturing or chipping in pseudotachylytes, and that the roundness of clasts may be used as a special index of disintegration attributed to attr ition and melting. (C) 1999 Elsevier Science Ltd. All rights reserved.