TEM and AEM constraints on the origin and significance of chlorite--mica stacks in slates: an example from Central Wales, U.K.*

-Chlorite-mica stacks (grains of intergrown chlorite and white mica) in a matrix of fine-grained white mica and chlorite have been studied using XRD, SEM, EMPA, TEM and AEM methods. The stacks occur in a weakly cleaved Llandoverian mudstone, central Wales, which has a white mica (illite) crystallinity index of 0.35 ° A20 corresponding to the lower anchizone. White mica occurs as packets (100/~ to 8 #m thick) interleaved with dominant chlorite packets in stacks, with both apparent coherent interfaces or cross-cutting small angle boundaries with chlorite layers. It is well-crystallized 2M~ polytype with phengitic composition and low paragonite component [Na/(Na+K) -< 0-0.07]. Chlorite in stacks is Fe-rich and relatively homogeneous. TiO2 crystals surround stacks and occur within chlorite, and white mica is Ti-rich compared to matrix white mica. Fine-grained white mica and chlorite in the matrix have two different orientations: one parallel to bedding and one parallel to cleavage, which is approximately 30-50 ° to bedding. Matrix white mica is predominantly a 2M1 polytype, but some cleavage-parallel white mica is 3 T and some bedding-parallel white mica is 1Md. It is Na-rich [Na/(Na+ K) 0.14-0.42] and relatively heterogeneous; some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation. Matrix white mica and chlorite each contain less Fe than corresponding white mica and chlorite in stacks. Both matrix and stack chlorite are one-layer polytypes. The data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals (probably via intermediate expandable trioctahedral phyllosilicates). Most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures. Subsequent deformation caused further modification of pre-existing chlorite-mica stacks whereas partial dissolution of stacks and bedding-parallel matrix phyllosilicates resulted in the formation of cleavage-parallel phyllosilicates. I N T R O D U C T I O N COARSE phyUosi l icate grains , each consis t ing of a s tack o f packe t s o f mica and ch lor i te a re c o m m o n in L o w e r Pa leozo ic m u d s t o n e s tha t have u n d e r g o n e d iagenes i s and low-g rade reg iona l m e t a m o r p h i s m in the W e l s h Bas in (A t t l ewe l l & T a y l o r 1969, B r e n c h l e y 1969, Evans & A d a m s 1975, F i t ches & Johnson 1978, Cra ig et al. 1982, W o o d l a n d 1982, 1985, D i m b e r l i n e 1986, Mi lodowsk i & Za las iewicz 1991, Li et al. 1992). T h e y usual ly consis t of in t e rg rowths of ch lor i te and whi te mica tha t a re gene ra l ly s t a cked para l l e l o r subpara l l e l to (001) p lanes , and are up to 150/~m in longes t d imens ion . Such * Contribution No. 494 from the Mineralogical Laboratory, Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, U.S.A. c h l o r i t e m i c a s tacks have been r e p o r t e d f rom m a n y o t h e r local i t ies in bo th low-grade m e t a m o r p h o s e d ( H o e p p e n e r 1956, Vol l 1960, K o s s o v s k a y a & Shutov 1970, Wi l l i ams 1972, Ho leywe l l & Tull is 1975, Bay ly et al. 1977, B e u t n e r 1978, R o y 1978, W h i t e & K n i p e 1978, W e b e r 1981, M o r a d 1986) and u n m e t a m o r p h o s e d mudrocks (Pye & Kr ins ley 1983), a l though d i f fe ren t t e rms have been used to descr ibe t hem (see rev iew by van de r Plui jm & Kaars -S i jpes te i jn 1984). Va r ious origins have been sugges ted for the fo rma t ion of the c h l o r i t e m i c a s tacks , including: (1) s t ra incon t ro l l ed growth and d isso lu t ion of ch lor i te in def o r m e d de t r i t a l micas (Vol l 1960, van d e r Plui jm & Kaars -S i jpes t e i jn 1984); ( 2 ) s t r a i n c o n t r o l l e d ' p o r p h y r o b las t s ' f o r m e d dur ing m e t a m o r p h i s m (At t l ewe l l & Taylor 1969, W e b e r 1981); (3) p r e s e d i m e n t a t i o n , de t r i t a l