THE INFLUENCE OF STRAIN, LITHOLOGY AND STRATIGRAPHICAL DEPTH ON WHITE MICA (ILLITE) CRYSTALLINITY IN MUDROCKS FROM THE VICINITY OF THE CORRIS SLATE BELT, WALES - IMPLICATIONS FOR THE TIMING OF METAMORPHISM IN THE WELSH BASIN

Low and very low grade metamorphism in a mudrock-dominated succession, ranging from Upper Cambrian to Wenlock, is characterized by the white mica (illite) crystallinity technique. Kubler indices indicate that grade ranges from late diagenetic to low epizonal. The prograde reaction most easily observed by XRD study is the progressive ordering of K/Na micas; diagnostic 2M1 polytypic reflections become recognizable at 0.4-DELTA-degrees 2-theta and strengthen as grade advances. Grade generally correlates with thickness of overburden in the > 9 km thick succession. However, strain exercises additionally a profound influence on grade, and the distribution of strain was related to decollement resulting from inhomogeneities and ductility contrasts within the succession. Strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates. The distribution of metamorphic grade implies that the Lower Palaeozoic succession was augmented by several kilometres of Lower Devonian (Old Red Sandstone) strata at the metamorphic culmination. The culmination was syntectonic and represents an Acadian rather than an early diastathermal event. Subsequent dextral movement on the Bala Fault displaced the regional isocryst pattern by c. 4 km. Retrogressive smectite appears to be localized to the vicinity of the Bala Fault and is probably of Variscan age.