A prograde sequence of corrensite and chlorite in pelitic rocks of the diagenetic zone, anchizone, and epizone (illite crystallinity indices = 0.17–0.58°Δ2θ) of the Gaspé Peninsula, Quebec, was studied by analytical and transmission electron microscopy (AEM and TEM). The data collectively suggest that diagenesis/metamorphism of chlorite and corrensite follows a sequence of phase transitions, compositional homogenization, and recrystallization, approaching a state of equilibrium for which chlorite is the stable phase.

Corrensite occurs as coalescing, wavy packets of layers intergrown with chlorite and illite in the diagenetic and low-grade anchizonal rocks. Intergrowths of discrete chlorite and corrensite crystals, interstratified packets of chlorite and corrensite layers, terminations of smectite-like layers by chlorite layers, and 2–3 repeats of R2- and R3-ordered chlorite-smectite mixed layers occur. These materials are alteration products of detrital biotite or other precursor phases like trioctahedral smectite. The crystal size and proportion of corrensite decrease significantly from the diagenetic zone to the anchizone. Deformed corrensite is crosscut by straight packets of chlorite and corrensite in the diagenetic sample. Some chlorite occurs as discrete, euhedral to subhedral crystals intergrown with or enclosed by other phases in the absence of corrensite. The crystal size of chlorite and definition of crystal boundaries increase whereas density of crystal imperfections and randomness in orientation decrease with increase in grade of diagenesis/metamorphism. Crystals that are kinked or bent, or display gliding along (001) form low-angle boundaries with relatively defect-free crystals, implying deformation during crystal growth. Abundant well-defined low-angle boundaries associated with dislocations are observed in the higher grade rocks, consistent with a stage of readjustment of crystal boundaries during crystal growth. The AEM analyses show that the corrensite has lower Fe/(Mg + Fe) and Al/(Si + Al) than the coexisting chlorite in the diagenetic sample, and that the ranges of composition of chlorite of different grades overlap and become smaller with increasing grade, implying prograde homogenization.

The data imply that corrensite is a unique phase that is metastable relative to chlorite: its conversion to chlorite occurred at a grade as low as that of the high-grade diagenetic zone. The textural relations suggest that the metamorphic crystallization and recrystallization were coeval with deformation processes due to tectonism, partially modified by subsequent contact metamorphism. The data, combined with those of previous reports, suggest that the Gaspé Ordovician rocks constitute a part of a regional distribution of trioctahedral phyllosilicate-rich rocks in the northern Appalachians. The regional occurrence of abundant chloritic minerals is thus directly related to a specific tectonic regime with precursor sediments largely derived from an andesitic arc system(s).

The clay mineral assemblages of the Tithonian and Berriasian sediments (Tera and Oncala Groups) in the eastern part of the Cameros basin are investigated at seven localities. The lowest-grade assemblage, located on the southern border of the basin, contains calcite + quartz + hematite + kaolinite + mixed-layer illite-smectite (R = 1, 65 85% illite layers) + discrete illite (IC = 0.5 0.65Δ°2θ). Systematic increases in the illite and chlorite crystallinities suggest increasing metamorphic grade from the northwest part of the basin to the southeast. This trend does not follow the pattern previously described for the overlying late Berriasian–early Aptian sediments (Urbión and Enciso Groups), which exhibit a higher metamorphic grade. This may result from local variations in sedimentary facies, as well as the circulation of hot migratory fluids. Tertiary compression occurring long after the main metamorphic event is considered to be responsible for the enhanced illite and chlorite crystallinities measured in the SE extreme of the basin.

Clay mineral evolution in diagenetic and very low-grade metamorphic sedimentary rocks from the Maláguide Complex was studied using the following parameters: illite and chlorite crystallinity, b cell dimension of illite, chemistry of chlorite, and the occurrence and distribution of the following phyllosilicates: I-S mixed-layer minerals, kaolin polytypes, chlorite-bearing mixedlayer minerals (tosudite and mica-chlorite interstratifications), vermiculite-like minerals, paragonite and biotite. The distribution of IC values permits the subdivision of the study area into three zones: diagenetic, anchizone and epizone. On the contrary, the distribution of CC values cannot be used in the study of the metamorphic zonation. The mineralogy and b dimension of illite indicate that the Maláguide samples display a low to low-medium pressure metamorphism, also characterized by steep temperature gradients.

The correlation between IC data and distribution of other significant minerals has permitted the establishment of a zonation within the chlorite zone, which is confirmed by the appearance of some index minerals, such as paragonite, biotite, chloritoid and andalusite.

The mafic rocks from boreholes studied here contain the assemblage typical for upper-subgreenschist to lower-greenschist facies (albite, chlorite, illite, titanite, quartz, Ti-oxide ± actinolite) as well as relict clinopyroxene.Metapelites underlying a metabasic rock sequence also record metamorphic alteration as demonstrated by the chlorite and illite crystallinities. Chlorite crystallinity suggests formation in conditions close to the subgreenschist/greenschist facies boundary, but this mineral is partly decomposed during metamorphic retrogression.

Analysis of pelites with detrital white-micas in the Clew Bay–Galway Bay segment of the Irish Caledonides indicates that b0 data from whole-rock and < 2 μm fractions generally show differences smaller than the errors of the method, irrespective of (001) illite crystallinity values, probably due to metamorphic recrystallization. Intermediate pressure metamorphism of the Ordovician–Silurian Clew Bay Group indicates slow subduction, allowing partial thermal re-equilibration before exhumation. In contrast, the Croagh Patrick Group Laurentian shelf-sediments underwent high-pressure alteration, suggesting rapid subduction/exhumation, synchronous with strike-slip faulting. The Murrisk Group, which underwent high-intermediate pressure metamorphism in an Ordovician back-arc, forms a separate terrane to the Croagh Patrick Group to the north and also to the Ordovician Lough Nafooey and Tourmakeady groups and Rosroe Formation in the south, in which low-intermediate pressure alteration occurred. These, together with the Silurian North Galway Group, may have undergone heating due to movement over or deposition on the hot Gowlaun Detachment as the Connemara Dalradian was exhumed. The South Connemara Group also underwent a high-pressure alteration, consistent with its inferred subduction environment. Evidence of contact alteration, due to known or inferred buried late- to post-Caledonian granitoid plutons, has been found in the Clew Bay, Louisburg–Clare Island, Croagh Patrick, Murrisk and South Connemara groups. These show evidence of lower-pressure alteration than the surrounding country-rocks.