Chlorite-mica stacks in low-strain rocks from central Wales

Abstract

Weakly deformed, low grade, Lower Palaeozoic metasediments from central Wales contain abundant stack-like intergrowths of chlorite and white mica that closely resemble stacks described from the Devonian Hunsruckschiefer of West Germany; the Ordovician Martinsburg Slate, New Jersey, U.S.A.; and elsewhere. Several theories have been proposed to explain the origin of such stacks, including a detrital origin; strain-controlled growth of chlorite on a detrital mica nucleus; and strain-controlled intergrowth during metamorphism. None of these satisfactorily explains the central Wales stacks. A detrital origin is precluded by the presence of many stacks with shapes too delicate to have survived transportation, and a lack of hydrodynamic equivalence between the stacks and the clastic host grains. Features inconsistent with strain-controlled growth are constant alignment parallel to bedding but non-systematic orientation with respect to tectonic cleavage, their common occurrence in undeformed rocks, and petrographic evidence that they precede the tectonic cleavage. It is proposed that the stacks formed during diagenesis and low-grade metamorphism, and before the onset of deformation, through mimetic growth on a primary bedding fabric composed of clay minerals.

Chemistry of Detrital Biotites and Their Phyllosilicate Intergrowths in Sandstones

“…Also the transformation of illite into muscovite and the increasing iron content in chlorites have been related to increasing temperature-pressure conditions (e.g., White et al, 1985). The results of the present study and those of other authors (e.g., Craig et al, 1982;White et al, 1985) suggest that alteration of biotite during diagenesis is a potential source of K and/or (Fe + Mg) for the authigenesis of illites and chlorites.…”

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

“…Grains with a modified detrital outline and consisting of subparallel packets of chlorite and white mica, referred to as chlorite-mica stacks, range from 10-I 50 /zm in size and are dispersed throughout the matrix, along with occasional quartz and albite clasts. The occurrence and origin of the chlorite-mica stacks have been studied by many workers (e.g., Craig et al, 1982;Woodland, 1985;Dimberline, 1986;Milodowski and Zalasiewicz, 1991;Li et al, 1992b). Li et al (1994) concluded that they were largely detrital biotite grains, subsequently modified during the progression from diagenesis to low-grade metamorphism.…”

Microstructures, Mixed Layering, and Polymorphism of Chlorite and Retrograde Berthierine in the Kidd Creek Massive Sulfide Deposit, Ontario

“…In clastic sedimentary rocks, phyllosilicate stacks consisting ofinterstratified parallel or subparallel packets of two different phyllosilicate phases have been shown to originate from detrital phyllosilicate grains, with or without recrystallization, in response to diagenetic/metamorphic conditions (e.g., Roy, 1978, Craig et al, 1982van der Pluijm and Kaars-Sijpesteijn, 1984;Gregg, 1985;Woodland, 1985;Dimberline, 1986;Morad, 1986). Deformation features due to tectonic stress are commonly observed in such stacks.…”