Volume loss and metasomatism during cleavage formation in carbonate rocks

Abstract

Controversy exists regarding the amount of volume loss associated with cleavage development. A combination of geometrical and geochemical techniques provides evidence for contemporaneous passive concentration (volume loss) and metasomatism leading to the formation of cleavage in carbonate lithologies and emphasizes the relationship between observational scales and textural heterogeneity. Comparison of protoliths and weakly- to strongly-cleaved rocks along steep, layer-parallel strain gradients within the Doublespring duplex, Lost River Range, Idaho enables quantitative assessments of geometric transformation and elemental mass transfer accompanying progressive deformation. Geometric finite strain analysis documents shortening in the Z direction at low strains ( ε̄ s<0.15) and in both the Z and Y directions at higher strains ( ε̄ s>0.15). Unbalanced shortening in Z and Y with extension in X results in volume loss. Geometrically derived volume strain estimates at the 1–4 cm 3 scale indicate volumetric dilations of from −2% to −12% in uncleaved to weakly-cleaved rocks, and from −20% to −50% in strongly- to very strongly-cleaved rocks. Small-scale significant volume losses and gains contrast with the estimated volume loss of <3% associated with duplex formation at the 100 km 3–1 km 3 scale due to the localization of strain within thin deformation zones. Sampling scale is also a factor in evaluating the major and trace element and O and C isotope compositions of the variably deformed rocks. Samples collected at the 4–15 cm 3 scale reflect mixtures of chemically and mineralogically distinct selvage and microlithon domains. Microdrilling of samples at scales of a few cubic millimeters to 1 cm 3 affords examination of chemical and isotopic differences among individual selvages and microlithons. The data for microdrilled samples demonstrate depletion of Ca and 18O and enrichment of K, Al, Si, Ti, Fe, Na, Mg and P in selvages relative to microlithons and nearby undeformed carbonate layers. The enrichments of some elements (Mg, P, Na and possibly Fe) are more compatible with a passive concentration mechanism, whereas the concentrations of other elements, particularly K, Al, Si and Ti, require significant metasomatic addition during deformation. Shifts in O-isotope compositions of deformed samples relative to the protolith δ 18O values unequivocally demonstrate open-system behavior during deformation. Trends of deformed samples toward lower δ 18O with increasing strain and the δ 18O values of veins are compatible with infiltration by H 2O-rich fluids with relatively low δ 18O V-SMOW (near 0‰). The formation of disjunctive cleavage at the Doublespring duplex occurred by passive concentration resulting from calcite dissolution and related volume loss, and neo-crystallization of illite+kaolinite+quartz±anatase in selvages as a result of metasomatic additions. Chemical and geometric strain softening within selvages likely led to enhanced and preferential fluid flow in selvages.