Static recrystallization and preferred orientation of phyllosilicates: Michigamme Formation, northern Michigan, USA

The Michigamme Formation of the Marquette District in Michigan's Upper Peni nsula comprises a sequence of cleaved rocks of increasing metamorphic grade . Because metamorphism in the area occurred after cleavage formation, the r ocks provide an opportunity to study preferred orientation development of p hyllosilicates under conditions of static recrystallization. X-ray texture goniometry an samples from the greenschist-facies zone that w ere collected at varying distances from the bounding biotite-in and garnet- in isograds, shows that: (1) the preferred orientation of phyllosilicates i s always parallel to the mesoscopic cleavage, and (2) the degree of preferr ed orientation of phyllosilicates improves as a function of increasing meta morphic grade (from <4 to >9 m.r.d.). Scanning electron microscopy on these samples shows that: (1) the length/width ratio increases with increasing g rade, and (2) grain shapes are better defined with increasing grade. Previous work on slates showed mechanical processes dominate at very low-gr ade metamorphism whereas chemical processes are favored at higher grades. T he Michigamme samples show that improvement of preferred orientation occurr ed by grain dissolution and crystallization. Noncleavage-parallel phyllosil icate grains were preferentially dissolved, probably facilitated by interna l strain energy from mineral defects, aided by chemical energy, whereas cle avage-parallel phyllosilicates were hosts for new growth along their basal planes. These results show that significant fabric strengthening can be ach ieved by grain dissolution and crystallization in the absence of tectonic s tress. (C) 2001 Elsevier Science Ltd. All rights reserved.