MICROTEXTURE OF SLATE FROM NORTHERN TAIWAN

Slaty rocks from northeastern Taiwan are examined on their microtexture. The original sediment in the slate belt is thought to be Miocene to Eocene in age and was metamorphosed during one or more episodes of arc-shelf collision which started at least 4 m.y. ago. Along a traverse perpendicular to the zone of collision, a major unconformity was suggested in the slate belt based on the appearance of assumed basal conglomerate and the lack of fossil in the lower Eocene formation. However, this episode is not supported by petrologic difference across the unconformity.

Detail microtexture examination down to micron scale in both the slaty formation and the quartzite layers within the slate formation failed to detect any significant difference on the degree of metamorphism. From the top most Miocene strata to the thickest part of the Eocene strata, the dominant structure is the sheared surface, which define the slaty cleavage, separated by quart/feldspar/mica lens. This structural style does not support the two stages of metamorphism separated by the basal conglomerate. Rather, it supports one episode of metamorphism of argillaceous sediments accumulated on the continental shelf since Eocene (?)

Observations from several traverses suggest that commonly observed crenulation (S2) upon S1is only locally developed in related to fault. At one critical locality, this type of fault caused a diversion of stream channel. The fact that the development of S2 is so closely related to the modern geomorphologic change suggests that the timing of S2 development is very close to the geologic recent.

Another prominent petrographic observation is that multiple overlapping, tight isoclinal fold is a rather common feature in slate as it is shown by traceable sandy laminar of 0.1 mm or thinner. This feature suggests buckling and layer perpendicular stress, rather than the layer paralleling shearing are probably the major mechanism in the creating the slaty cleavage (Ez, 2000).