Systematic periodicity of glaucophane metamorphism: An illusion or an important geologic regularity

New data on metamorphic complexes in Japan, the Koryak Highland in NE Russi a, Sakhalin, California, and other areas suggest that the systematic period icity in the occurrence of glaucophane metamorphism is an important geologi cal regularity rather than an illusion. In Japan, the general age sequence of glaucophane schists, which were first formed late during the development of the Maizuru ophiolites and early during the episode in the Renge belt ( at approximately 360 Ma), was repeated at 30 m.y. intervals: at 360, 330, 3 00, 250, 220, 185, 150, 120, and 90 Ma. These metamorphic maxima are detect ed in different belts and are commonly separated by pulses of the origin of ophiolites. Tectonic slabs in the system of nappes of different ages in th e Koryak Highland usually carry either glaucophane schists or melange with glaucophane-schist and eclogite fragments in the soles, and the local metam orphic maxima are generally the same as in Japan. The geologic history of t he North American Cordilleras involves a number of stages of accretion, thr usting, and exhumation of glaucophane schists, which have ages of 220, 175, 120, 80-90, and 55-60 Ma. Older stages at 410 and 260 Ma were detected in Alaska. Traces of more ancient pulses with ages ranging from 650 to 330 Ma were detected in Paleozoic belts in Central Asia, Australia, in the Urals, and in western Europe. The histogram of Fig. 9, which summarizes all availa ble data on the age of glaucophane-schist metamorphism, confirms the above- mentioned systematic periodicity. The possible causes of the wide age inter vals, which span two or three adjacent maxima in some belts, are discussed, along with the examples of polyphase evolution in the Alps and the Sanbaga wa belt and the quantitative P-T-t estimates of the metamorphic evolution. The proofs and causes of the fast and superfast exhumation of glaucophane-s chist and eclogite slabs are discussed and the possibility of the irreversi ble Earth's evolution, in particular, subduction processes, is considered.