ELSEVIER Lithos, 33 (1994) 119-133 LITHOS Evolution of the Sambagawa metamorphic belt, Japan A. Takasu a, S.R. Wallis b, S. Banno b, R.D. Dallmeyer c aDepartment of Geology, Shirnane University, Matsue 690, Japan bDepartment of Geology and Mineralogy, Kyoto University, Kyoto 606, Japan CDepartment of Geology, Universityof Georgia, Athens, GA 30602, USA Received 20 December, 1992; revised and accepted 15 August, 1993 Abstract The high-pressure Sambagawa metamorphic belt evolved within a Mesozoic accretionary prism that developed in the convergent margin along the east of the Eurasian continent. Although extensively researched, recent studies in this region have revealed a number of important features that allow more complete reconstruction of the tec- tonic history. (i) The grouping of radiometric ages suggests the existence of three major structural units with distinct metamorphic histories. One of these has since been completely eroded away. (ii) The difference of ages recorded by different minerals is, in many cases, small suggesting rapid exhumation after reaching peak metamor- phic conditions. (iii) The dominant ductile deformation of the Sanbagawa belt representing major orogen-parallel flow is commonly related to retrograde metamorphic reactions. This shows that the associated fabric developed during exhumation not subduction. (iv) Kinematic studies suggest that the main ductile deformation caused ma- jor ductile thinning of the region which was probably a significant factor in causing exhumation of the region. 1. Introduction The Japanese Islands record a complex tec- tonic evolution reflecting the progressive devel- opment of accretionary wedge complexes along the eastern margin of the Eurasian continent at least since the late Paleozoic age. The Samba- gawa (Sanbagawa) belt is a glaucophanic meta- morphic belt, and the metamorphic sequences range from the pumpellyite-actinolite facies and the glaucophane schist facies through the epi- dote amphibolite facies, and locally up to the eclogite facies. The study of such regional meta- morphic rocks helps to develop a quantitative understanding of orogenic processes in the deeper parts of convergent margins. Phase petrology, isotope geochronology and kinematic studies are some of the most important tools in trying to de- termine P-T-t-D (deformation) paths of indi- vidual metamorphic rocks and metamorphic complexes as a whole. The Sambagawa belt is one of the most extensively studied metamorphic complexes in Japan (cf. Wallis and Banno, 1990), and offers a clear example of a metamor- phic complex which developed in a subduction zone. The main purpose of this paper is to empha- size the recent developments in the study of the Sambagawa belt, in particular emphasizing the geochronology and kinematic studies. Com- bined with earlier results, these studies provide a framework for understanding the tectonic evo- lution of the Sambagawa accretionary prism. The Shikoku region is the best exposed and most in- tensively studied part of the Sambagawa belt and we shall concentrate our discussion on this re- gion. Although variation exists along the 800 km 0024-4937/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0024-4937 ( 94 ) 00022-T