STRAIN-RELATED DIFFERENCES IN THE CRYSTAL-GROWTH OF WHITE MICA AND CHLORITE - A TEM AND XRD STUDY OF THE DEVELOPMENT OF METAPELITIC MICROFABRICS IN THE SOUTHERN UPLANDS THRUST TERRANE, SCOTLAND

TEM and XRD techniques were used to study crystal growth characteristi cs of the fabric-forming phyllosilicates which developed in response t o low-grade metamorphism and tectonic imbrication in part of the South ern Uplands thrust terrane. Prograde regional metamorphism, ranging fr om late diagenesis through the anchizone to the epizone, was accompani ed by the development of a slaty cleavage which is commonly bedding-pa rallel, TEM-measured mean thicknesses of white mica and chlorite cryst allite populations increase with advancing grade and correlate with XR D-measured crystallinity indices. Analytical TEM data show that progra de changes in composition lead to a net loss of Si, Ca and minor Fe fr om the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 Angstrom, and indi cate an intermediate pressure series of metamorphism with a field grad ient of < 25 degrees C km(-1). Chlorite compositions evolved from diab antite (with intergrown corrensite) to ripidolite over an estimated te mperature range of 150-320 degrees C. Field gradient and temperature e stimates suggest that crystal growth and fabric development occurred a t burial depths ranging from 6 km to at least 13 km in the thrust terr ane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, a nd resulted in similar size distributions which resemble typical Ostwa ld ripening curves. Under anchizonal and epizonal conditions, white mi ca grew more rapidly than chlorite because of its greater ability to s tore strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. I n contrast, chlorite crystals which grew under these conditions develo ped subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain-induced crystal growth indi cate that white mica (illite) and chlorite crystallinity indices are l ikely to show significant differences where low-grade metamorphism is closely associated with tectonic fabric development.