VOLUME LOSS AND METASOMATISM DURING CLEAVAGE FORMATION IN CARBONATE ROCKS

Controversy exists regarding the amount of volume loss associated with cleavage development. A combination of geometrical and geochemical te chniques provides evidence for contemporaneous passive concentration ( volume loss) and metasomatism leading to the formation of cleavage in carbonate lithologies and emphasizes the relationship between observat ional scales and textural heterogeneity. Comparison of protoliths and weakly- to strongly-cleaved rocks along steep, layer-parallel strain g radients within the Doublespring duplex. Lost River Range, Idaho enabl es quantitative assessments of geometric transformation and elemental mass transfer accompanying progressive deformation. Geometric finite s train analysis documents shortening in the Z direction at low strains (epsilon(s) < 0.15) and in both the Z and Y directions at higher strai ns (epsilon(s) > 0.15). Unbalanced shortening in Z and Y with extensio n in X results in volume loss. Geometrically derived volume strain est imates 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 v olume 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 t o the localization of strain within thin deformation zones. Sampling s cale is also a factor in evaluating the major and trace element and O and C isotope compositions of the variably deformed rocks. Samples col lected at the 4-15 cm(3) scale reflect mixtures of chemically and mine ralogically distinct selvage and microlithon domains. Microdrilling of samples at scales or a few cubic millimeters to 1 cm(3) affords exami nation of chemical and isotopic differences amen individual selvages a nd microlithons. The data for microdrilled samples demonstrate depleti on of Ca and O-18 and enrichment of K, Al, Si, Ti, Fe, Na, Mg and P in selvages relative to microlithons and nearby undeformed carbonate lay ers. The enrichments of some elements (Mg, P, Na and possibly Fe) are more compatible with a passive concentration mechanism. whereas the co ncentrations of other elements, particularly K, Al, Si and Ti, require significant metasomatic addition during deformation. Shifts in O-isot ope compositions of deformed samples relative to the protolith delta(1 8)O values unequivocally demonstrate open-system behavior during defor mation. Trends of deformed samples toward lower delta(10)O with increa sing strain and the delta(18)O values of veins are compatible with inf iltration by H2O-rich fluids with relatively low delta(18)O(V-SMOW) (n ear 0 parts per thousand). The formation of disjunctive cleavage at th e 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 selvage s. (C) 1998 Elsevier Science Ltd. All rights reserved.