Non-volatile element and volume flux in coalesced slaty cleavage

Abstract

Non-volatile element and volume flux during slaty cleavage formation was determined by mapping major element compositions in hand specimens of slate with variable cleavage intensity due to non-argillaceous interlayers. Intense spaced cleavage zones form where cleavage coalesces within the inner arcs of folded nonargillaceous layers or at the offsets between imbricated non-argillaceous layers. The element distributions in samples from Nova Scotia, Vermont and New Jersey were mapped using an XRFmacroprobe, an energy dispersive XRF with automated spectrum acquisition and a mm-scale collimated beam. Cleavage zones are depleted in quartz and SiO 2, and enriched in phyllosilicates, Al 2O 3, Fe 2O 3, K 2O, MgO and TiO 2. CaO and MnO show more variable behavior consistent with simultaneous depletion in carbonates and residual enrichment in silicates. Quartz-rich, weakly cleaved areas adjacent to fold outer arcs or between imbricate offsets are enriched in quartz with respect to probable pre-cleavage compositions, with SiO 2 commonly reaching 75–80 wt%. The spaced cleavage zones and their intervening microlithons grade texturally and compositionally into penetrative cleavage with distance from non-argillaceous layering. Average compositions of heterogeneously cleaved beds are quite similar to compositions of homogeneously cleaved beds in the same sample, suggesting balanced, localized element enrichment and depletion adjacent to non-argillaceous layers. The similarities between average macroprobe, slate and shale compositions suggest minimal non-volatile net volume flux during the formation of slaty cleavage. Volume losses of 50% by SiO 2 flux would result in a rock with approximately equal SiO 2 and Al 2O 3 wt%, not a slate. Most of the non-volatile element and volume flux in the slates occurred on a hand specimen scale, with quartz depletion in cleavage zones balanced by enrichment in adjoining microlithons.