A NEW ESTIMATE OF THE CONDITIONS FOR TRANSITION FROM BASAL[A] TO PRISM[C] SLIP IN NATURALLY DEFORMED QUARTZ

Quartz c-axis fabrics of naturally deformed quartz in metacherts from the Ryoke metamorphic belt in the Yanai district, southwestern Japan, suggest that the prism [c], prism [a] and basal [a] slip systems were activated during the plastic deformation under high-temperature metamo rphic conditions. With increasing temperature, quartz c-axis fabric pa tterns change from girdle type, through X-maximum with a faint crossed girdle, to X-maximum. The dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temper ature, closely correlated with the change in quartz c-axis fabric. The fabric transition is also accompanied by an abrupt increase of recrys tallized quartz grain size. Moreover, observations with transmission e lectron microscopy (TEM) clearly identify the dominant activation of [ c] slip through the presence of basal subgrain boundaries and free dis locations with the [0001] Burgers vector in the quartz from the metach ert samples exhibiting X-maximum c-axis fabric. Based on these observa tions it is concluded that the mechanism switch from basal [a] to pris m [c] slip systems occurred with increasing temperature in the metache rts. The transition temperature is petrologically estimated to be ca. 550-600 degrees C at the natural strain rate.