Deformation volume and cleavage development in metasedimentary rocks from the Ballarat slate belt

doi:10.1016/0191-8141(88)90127-7

Journal of Structural Geology

Volume 10, Issue 1, 1988, Pages 53-62

https://doi.org/10.1016/0191-8141(88)90127-7 Get rights and content

Abstract

Psammites from the Ballarat slate belt in SE Australia exhibit well-developed differentiated or spaced cleavage defined by alternating phyllosilicate- and quartz-rich domains (termed P- and Q-domains, respectively). Strain estimates derived from independent microstructural and chemical observations suggest that the P-Q fabrics developed in response to plane-strain deformation dominated by solution transfer with the principal finite shortening in the P-domains approximately twice that in the adjacent Q-domains. Significant finite extensions are indicated by ubiquitous quartz-albite-chlorite overgrowths in both P- and Q-domains, while pressure-shadow development around syntectonic pyrite porphyroblasts suggest finite extension of at least 100%. This estimate is comparable with the extension predicted for constant-volume deformation and consequently there appears to have been no significant bulk material loss or gain on the hand-specimen scale. These observations are consistent with solute transfer scales as little as a few centimetres via diffusion in a stationary fluid and do not require, although do not necessarily preclude, large-scale advective fluid transport through the slate belt during cleavage formation as suggested in previous studies.

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Here, we study slate microfabrics from the exhumed accretionary wedge of the central European Alps and focus on the development of foliation. High-resolution micrographs from novel BIB-SEM imaging and Synchrotron X-ray Fluorescence Microscopy are analysed with 2D auto-correlation functions to quantify the geometry and spacing of slate microfabrics along a metamorphic gradient covering the outer and inner wedge (200–330 °C). The sedimentary layering primarily controls the morphology of the slate microfabrics. However, from outer to inner wedge, a fabric evolution is observed where diagenetic foliations gradually transform to secondary continuous and spaced foliations. With increasing metamorphic grade, the amount of recrystallized phyllosilicate grains and their interconnectivity increase, as does clast/microlithon elongation (aspect ratios up to 11), while foliation spacing decreases to <20 μm. This foliation evolution under non-coaxial deformation involves a combination of mechanical rotation of phyllosilicates, fracturing, and fluid-assisted pressure-dissolution-precipitation creep. The latter is the dominant deformation mechanism at T > 230 °C and accommodates background strain in the inner wedge. The evolving microstructural anisotropy is interpreted to lead to strain weakening by structural softening and may provide preferential fluid pathways parallel to the foliation, enabling the dehydration of large rock volumes in accretionary sediment wedges undergoing prograde metamorphism.
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Clasts in the YZ plane are tailed by poorly developed quartz ribbons that have limited extent in the foliation, in contrast to the XZ plane that shows ribbons as long as, or longer than the thin section. These relations suggest a pattern of monoclinic deformation (Waldron and Sandiford, 1988; Passchier, 1998). In addition, finite strain analysis on the metatillite and phyllite in the Zhangbaling belt indicates that deformation is nearly plane strain with slight flattening strain at the base and slight constriction at the top in our previous work (Zhang et al., 2007).
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The initial amount of clay/phyllosilicates and their sedimentary distribution/arrangement initially present in the rock will determine cleavage spacing (Fueten et al., 2002) and the amount of silica that can be locally mobilized from one area of the rock to another. Thus in our conceptual model (Fig. 9a), the original clay- and chlorite-rich sedimentary laminae of the Puncoviscana Formation preferentially developed a disjunctive cleavage parallel to bedding, Sp, in anchizone rocks and became further transformed at greenschist facies into alternating quartz-poor and quartz-rich compositional banding, similar in appearance to the classic P-Q fabrics of Waldron and Sandiford (1988), the ‘type C’ rough cleavage defined by Gray (1978) and Onasch (1983), and the zonal cleavage described by Murphy (1990). Our results support the model of Fueten et al. (2002) in which disjunctive cleavage can develop as a result of rheological instability in a rock, which in our examples is an initial, relatively regularly spaced compositional variation.
We investigate the influence of sedimentary structures in development of foliations in banded psammites and pelites in the meta-turbiditic Pampean Belt, Eastern Sierras Pampeanas, northwest Argentina. These anchizone to upper greenschist facies rocks were exhumed from near surface to mid-crustal levels, and preserve sedimentary structures and gradual changes in tectonic fabrics and microstructures. Primary variations in clay content affected the degree of development of a compaction-related pressure-solution cleavage (S_P) resulting in enhancement of the original sedimentary banding. The resulting compositional banding was overprinted by a tectonic cleavage (S₁) related to mid-Cambrian chevron folding. In banded sandstones, pressure-solution and mica growth that define S_P and S₁ foliations are increasingly better developed in phyllosilicate-rich domains with increasing structural depth and metamorphic grade. Clay-rich laminae in the protolith acted as the locus for mica nucleation and growth, favoring dissolution of quartz (up to 42%) causing passive concentration of mica. Pressure-solution thus produced banded sandstones with a spaced, bedding-parallel S_P cleavage in shallower sections and compositionally banded psammites and schists at depth. These observations demonstrate that formation of compositional banding does not require tectonic transposition of an older planar fabric. Similar processes may produce compositional banding in a wide range of meta-sedimentary rocks.
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High-resolution compositional maps provide a new tool for investigating mass transfer during cleavage formation. The Moretown Formation of western Massachusetts contains a well-developed crenulation cleavage with alternating mica-rich crenulation limbs and mica-poor crenulation hinges. Compositional mapping shows two generations of plagioclase, the second of which was synchronous with the crenulation cleavage. A significant amount of the syn-crenulation plagioclase (10–20% modally) grew in hinge domains. A small amount of syn-crenulation plagioclase (∼1%) and a large amount of phengitic muscovite grew in limb domains. The maps also show that uncrenulated domains experienced mass transfer and reactivation of older cleavages, and thus cannot be used as ‘undeformed’ reference domains for comparison with crenulated regions. Compositional mapping facilitates a new degree of integration between petrologic and structural analysis. Knowledge of the structural context of compositional domains allows better selection of phases and compositions for interpreting metamorphic reactions and linking metamorphism to deformational stages. Knowledge of syntectonic reactions provides new insights into mass transfer and volume change during deformation. In the Moretown Formation, plagioclase- and phengite-producing reactions play a large role in controlling the nature and magnitude of mass transfer, but microstructures control the location of reactants and products within the evolving fabric.

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^∗: Present address: Becquerel Laboratories Pty Ltd, P.O. Box 93, Menai, N.S.W. 2234, Australia.

^†: Present address: Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, S.A. 5001, Australia.

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Deformation volume and cleavage development in metasedimentary rocks from the Ballarat slate belt

Abstract

Lithos

J. Struct. Geol.

J. Struct. Geol.

Chem. Geol.

Tectonophysics

Chem. Geol.

Tectonophysics

J. Geochem. Explor.

J. Struct. Geol.

Chem. Geol.

J. Struct. Geol.

Earth Planet. Sci. Lett.

Classification of solution cleavage in pelagic limestones

Geology

Slaty cleavage and related strain in Martinsburg Slate, Delaware Water Gap, New Jersey

Am. J. Sci.

Oolite deformation in the South Mountain fold, Maryland

Bull. geol. Soc. Am.

Ocean floor metamorphism in the Betts Cove ophiolite

Contr. Miner. Petrol.

Solution-transfer, an important geological deformation mechanism

Nature, Lond.

Mechanisms for strain within the upper Devonian clastic sequence of the Appalachian plateau, Western New York

Am. J. Sci.