Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Abstract

Abstract-A prograde sequence ofcorrensite and chlorite in pelitic rocks of the diagenetic zone, anchizone, and epizone (illite crystallinity indices = 0.17-0.58~ of the Gasp6 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 A1/(Si + A1) 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 Gasp60rdovician 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 re#me with precursor sediments largely derived from an andesitic arc system(s).

Chlorite, Corrensite, and Chlorite-Mica in Late Jurassic Fluvio-Lacustrine Sediments of the Cameros Basin of Northeastern Spain

“…The analyses of corrensite are similar to those of Inoue (1985), Shau et al (1990), Shau and Peacor (1992), Jiang and Peacor (1994b), and Schiffman and Staudigel (1995), with higher Si and Na + K + Ca contents and slightly lower values of Fe/(Fe + Mg) than chlorites. Higher substitutions of octahedral Fe 2+ for Mg 2+ in the chlorites may compensate for the distortions introduced by the substitution of the larger A13+ cation for Si 4+ in the tetrahedral sheet (Shau et al, 1990).…”

“…Hillier (1993) suggested that coexisting Mg and Fe-rich chlorites within a diagenetic mudrock are indicative of disequilibrium at the microscopic scale. In contrast, Jiang and Peacor (1994b) and Jiang et al (1994) described heterogeneous chlorite compositions in diagenetic and anchizonal samples, which become progressively more homogeneous in the epizone. In our study, metamorphic conditions estimated from IC and ChC data are between anchizone and epizone, which is consistent with the observed homogeneity in chlorite compositions within a given sample.…”

Chlorite Geothermometry?—Contamination and Apparent Octahedral Vacancies

“…The first includes five pelitic rocks (A419, A416, R299, 89-12, and R245) as part ofa prograde sequence ranging from the diagenetic zone to the epizone (illite crystallinity indices = 0.19-0.58 ~ A20 for ethylene glycol solvated < 2 #m fractions) in the Gasp6 Peninsula, Quebec (Jiang and Peacor 1994). The diagenetic rocks (samples A419 and A416) contain discrete chlorite, and corrensite and illite that locally occur as intimate intergrowths with chlorite (Figure 1 a).…”

Low-grade metamorphism of Cambro-Ordovician successions in the Famatina belt, Southern-Central Andes: Burial-inversion history linked to the evolution of the proto-Andean Gondwana margin

“…Interstratified Chl/Vrm present in some samples could correspond to a metaestable phase from which chlorite formed, being part of the prograde reaction series corrensite→corrensite-chlorite→chlorite. The presence of intermediate phases belonging to this series, in which the vermiculite would be generated by detrital biotite alteration (Bozkaya and Yalçin, 2004), is consistent with late diagenetic conditions (Jiang and Peacor, 1994;Merriman and Peacor, 1999). However, some of the samples containing interstratified Chl/Vrm show KI (CIS) values corresponding to high-anchizone conditions (e.g., Achavil Formation), under which Chl/Vrm is generally not stable, and it seems more likely they are the product of retrograde reactions.…”