The Diagenetic to Low-Grade Metamorphic Evolution of Matrix White Micas in the System Muscovite-Paragonite in a Mudrock from Central Wales, United Kingdom

Abstract

Abstract--Two orientations of white micas with subordinate chlorite have been observed in a fine-grained (50 ~, to 2 ~tm) matrix of a Silurian lower anchizonal mudrock from central Wales: one parallel to bedding and one parallel to cleavage that is approximately 30*-50 ~ to bedding. Bedding-parallel micas consist of small (50-200 ~ thick) deformed packets (IMd polytype) and larger (100 ~-2 t~m) strain-free grains (2M~ polytype). All strained micas and some strain-free grains have compositions varying from Mus~Pg~4 to The data suggest that bedding-parallel metastable micas with disordered interlayer K and Na were initially derived from alteration of smectite during burial diagenesis. They subsequently underwent dissolution, with crystallization of more evolved bedding-parallel micas during deep burial. Discrete grains of stable muscovite and paragonite then crystallized in the cleavage orientation through tectonic stressinduced dissolution of bedding-parallel matrix micas. Combined XRD and TEM/AEM data further show that the so-called 6:4 ordered mixed-layer paragonite/muscovite actually corresponds to cation-disordered, homogeneous mica of intermediate composition.

Evolution of Illite/Smectite from Early Diagenesis Through Incipient Metamorphism in Sediments of the Basque-Cantabrian Basin

“…These authors observed transitions from illite-rich material with characteristics similar to those described above to mature micas and inferred that the transition represents a step (it is "episodic") triggered by tectonic activity. We conclude that the Basque-Cantabrian sequence is representative of the limits of passive burial metamorphism, at least insofar as transitions involving dioctahedral phyllosilicates are concerned, and that the deepest portions of the section overlap the conditions for tectonically-affected shale and slate sequences such as those described by Jiaug et al (1994) and Li et al (1994). For the BasqueCantabrian Basin, the burial temperature in the absence of tectonic stress could not cause any significant mineral changes, such as illite-phengite reaction, the 1Md --2M polytype transformation, or the change in crystal outline of SAED patterns, which have retained the turbostratic stacking order.…”

“…Prograde sequences have been studied from the Gasp6 Peninsula by Jiang et al (1994) and from Wales by Li et al (1994). They found that the lowest grades belong to the upper zone of diagenesis, extending through the epizone, and prograde changes were caused by tectonic stress.…”

“…The Gulf Coast sequence is typical, with transformation of smectite to illite-rich US over a narrow depth range, and with little or no subsequent change in clays after further burial, although Eberl (1993) discussed the possibility of changes in illite caused by Ostwald ripening. Mudstones of such dia-genetic grade may remain unchanged, for example, the 1.1 Gya-old Nonesuch shale of Michigan (Li et al 1994). Such sequences are generally the concern of sedimentary petrologists; and 2) Sequences involving changes caused by tectonic stress.…”

Dioctahedral mixed K-Na-micas and paragonite in diagenetic to low-temperature metamorphic terrains: bulk rock chemical, thermodynamic and textural constraints

“…Mixed-layering of micas was not observed. Studying Silurian low-T anchizonal mudrocks from Wales by XRPD, TEM and AEM methods, Li et al (1994) demonstrated that the so called 6:4 ordered mixed-layer Pg/Ms originally described by Frey (1978) corresponds to an interlayer cation-disordered, homogeneous white mica of intermediate composition. This mineral formed from smectite by burial diagenesis.…”

“…Although the steps of reaction progress of these phyllosilicates and their precursor phases have been understood fairly well (Livi et al 1990;Li et al 1994;Livi et al1997), little work has been published dealing with the distinctive cause of their occurrences, i.e. which metaclastites do or do not contain paragonite and its precursor phases.…”