1. metamorphic Geol., 1995, 13, 559-576

Strain-related differences in the crystal growth of white mica and chlorite: a TEM and X R D study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland R . 1. M E R R I M A N , ' B . ROBERTS,' D. R . PEACOR' A N D S. R . H I R O N S ' ' British Geological Survey, Keyworth, Nottingham NG12 5GG, UK (email: [email protected]) Birkbeck College, University of London, Malet Street, London WClE 7HX, UK University of Michigan, Ann Arbor, M I 48 109- 1063, USA

ABSTRACT

TEM and XRD techniques were used to study crystal growth characteristics of the fabric-forming phyllosilicates which developed in response to low-grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding-parallel. TEM-measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD-measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 A, and indicate an intermediate pressure series of metamorphism with a field gradient of